Source: Understanding color & the in-camera image processing pipeline for computer vision
Electromagnetic Spectrum
Source: Notes for the course of Color Digital Image Processing
Color Temperature
Source: Understanding color & the in-camera image processing pipeline for computer vision
Color Temperatures of the Stars
Luminosity Function
Source: Understanding color & the in-camera image processing pipeline for computer vision
CIE 1931 XYZ
Source: Understanding color & the in-camera image processing pipeline for computer vision
Luminance
Source: Human Vision and Color
Brightness, Lightness,Hue, Saturation, and Luminosity
Source: The Brightness of Colour
Brightness has been defined as the perceived intensity of a visual stimulus, irrespective of its source. Lightness, on the other hand, is defined as the apparent brightness of an object relative to the object’s reflectance. Thus increasing the intensity of light falling on an object will increase its apparent brightness but not necessarily its apparent lightness, other things being equal [1]. Saturation is a measure of the spectral ‘‘purity’’ of a colour, and thus how different it is from a neutral, achromatic stimulus. Hue is the perception of how similar a stimulus is to red, green, blue etc. Luminous efficiency, or luminosity, measures the effect that light of different wavelengths has on the human visual system. It is a function of wavelength, usually written as V(l) [2], and is typically measured by rapidly alternating a pair of stimuli falling on the same area of the retina; the subject alters the physical radiance of one stimulus until the apparent flickering is minimised. Thus luminance is a measure of the intensity of a stimulus given the sensitivity of the human visual system, and so is integrated over wavelength [3]. Luminance is thought to be used by the brain to process motion, form and texture [4].
Clearly, brightness is monotonically related to luminance in the simplest case: the more luminant the stimulus is, the brighter it appears to be. However, the Helmholtz-Kohlrausch (HK) effect shows that the brightness of a stimulus is not a simple representation of luminance, since the brightness of equally luminant stimuli changes with their relative saturation (i.e. strongly coloured stimuli appear brighter than grey stimuli), and with shifts in the spectral distribution of the stimulus (e.g. ‘blues’ and ‘reds’ appear brighter than ‘greens’ and ‘yellows’ at equiluminance) [1; 5–6].
The HK effect has been measured in a variety of psychophysical studies [7–8] and is often expressed in terms of the (variable) ratio between brightness and luminance.
followed by Black: The History of a Color (2009) and then Green: The History of a Color (2014), all produced by the same publisher. A fifth, devoted to yellow, should come next.
Historic Look on Color Theory
Steele R. Stokley
The evolution of colour in design from the 1950s to today
Francesca Valan
Journal of the International Colour Association (2012): 8, 55-60
In: Webvision: The Organization of the Retina and Visual System [Internet]. Salt Lake City (UT): University of Utah Health Sciences Center; 1995–.2005 May 1 [updated 2007 Jul 9]
Title: A Review of RGB Color Spaces …from xyY to R’G’B’
The CIE XYZ and xyY Color Spaces
Douglas A. Kerr
DIVERSE CELL TYPES, CIRCUITS, AND MECHANISMS FOR COLOR VISION IN THE VERTEBRATE RETINA
Wallace B. Thoreson and Dennis M. Dacey
Department of Ophthalmology and Visual Sciences, Truhlsen Eye Institute, University of Nebraska Medical Center, Omaha, Nebraska; and Department of Biological Structure, Washington National Primate Research Center, University of Washington, Seattle, Washington
Physiol Rev 99: 1527–1573, 2019 Published May 29, 2019; doi:10.1152/physrev.00027.2018
Are you new to blogging, and do you want step-by-step guidance on how to publish and grow your blog? Learn more about our new Blogging for Beginners course and get 50% off through December 10th.
WordPress.com is excited to announce our newest offering: a course just for beginning bloggers where you’ll learn everything you need to know about blogging from the most trusted experts in the industry. We have helped millions of blogs get up and running, we know what works, and we want you to to know everything we know. This course provides all the fundamental skills and inspiration you need to get your blog started, an interactive community forum, and content updated annually.
Source: Review of Display Technologies Focusing on Power Consumption
Electronic paper, popularly known as e-paper, can be defined as a dynamic display technology that emulates traditional paper. As LCD, e-paper belongs to the non-emissive display category but, in this case, no backlight is needed since the ambient light from the environment is enough.
The display is composed of millions of microcapsules containing positively charged white and negatively charged black particles suspended in a clear liquid, which are capable of producing the resolution only found in print. As they are bi-stable, they only consume power while the display is being updated. The power required for the update process depends on the size of the display.
The first commercial success of monochrome e-paper devices was due to the Electrophoretics technology, wrongly referred to as electronic paper displays (EPD), whose main exponent is microencapsulated electrophoretic displays, also known as e-ink [35]. Another similar approach, microcellular electrophoretic display films (SiPix), was bought by e-ink. There are other proprietary electrophoretic displays, which include Quick-Response Liquid Powder Display (QR-LPD) by Bridgestone [36], bichromal beads [37] by Xerox (Gyricon), or reverse emulsion electrophoretic display (REED) used by Zikon Corp.
Cholesteric liquid crystal (ChLCD), already mentioned as a subgroup of LCD, is generally classified as e-paper because of its zero consumption when it is not receiving screen updates.
Source: Review of Display Technologies Focusing on Power Consumption
A next generation of flexible, color and video e-paper is currently emerging. The most promising seems to be the electro-wetting approach (EWD) [38]. Its main component, liquavista (see Figure 3b), was developed by Philips but currently belongs to Amazon. Another interesting technology based on Interferometric Modulation (IMOD) is microelectromechanical sytems (MEMS) [39], whose potential has been demonstrated through several prototypes (trademarked Mirasol [40]) developed by Qualcomm.
Some other remarkable developments are the in-plane electrophoretics (IPE) patented by Canon [41] and HP’s Electrokinetic (EKD) [42], although they are at least several years away from a general market uptake [43].
Another less matured technology is electrofluidic [44], which presents the main novelty of using a three-dimensional microfluidic device structure and offering brilliantly colored aqueous pigment dispersions. Recently, other lines of investigation have aimed to simulate traditional paper through electronic paper made from microbial cellulose [45].
Source: Biological versus electronic adaptive coloration: how can one inform the other?
Nemoptic – BiNem/OLED dual Mode Display- Sold rights to Seiko Japan
ZBD Display – PM-LCD Reflective Display – Acquired by New Vision Display China
Fujitsu’s Color E-paper Mobile Display – FLEPia – Discontinued 2010
Reflective Displays using Pigments
Electrophoretic Displays
Electrowetting Displays
Electrochromic Displays
Electrophoretic Displays – Reflective
(eReaders and Note taking/Writing Pads)
(Monochrome and Color)
(EPD)
E ink
E Paper
Yotaphone
E Ink Triton
E Ink Triton 2
E Ink Kaleido
E Ink Kaleido 2
Hisense
Pocketbook
Onyx Boox Poke 2
Onyx Max Lumi
Quirk logic Papyr
Ratta Supernote A5X
Onyx Note Air
iReader
iFLYTEK
Amazon Kindle
Etch a Sketch
Magna Doodle
Remarkable 2
Ricoh Whiteboard 42 inch
Hisense A7 CC 6.7 Inch Smartphone
Kobo Libra H20
Amazon Paperwhite
Amazon Oasis
Bigme S3 7.8 Color E-reader
Kobo Nia e Readers
Boyue Likebook
Pocketbook Inkpad Color
Onyx Boox Poke 2 Color
Electrophoretic Displays – Reflective
Source: Stretchable and reflective displays: materials, technologies and strategies
Electrophoretic Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
In-plane Electrophoretic Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
ElectroKinetic Displays
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Liquid Powder Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Companies manufacturing Reflective Displays
The key players in the global e-paper display market are
Displaydata Ltd. (UK)
Display Innovations (UK)
Kent Displays, Inc. (USA)
LANCOM Systems GmbH (Germany)
Liquavista B.V. (The Netherlands)
Xerox Corp. (USA)
Zikon, Inc. (USA)
Qualcomm
Gamma Dynamics
ITRI
GUANGZHOU OED TECHNOLOGIES CO., LTD
InkCase Enterprise Pte Ltd
Plastic Logic HK Ltd
GDS Holding S.r.l.
Epson Europe Electronics GmbH
GDS S.p.a
Motion Display
MPicoSys Low Power Innovators
Omni-ID
Solomon Systech
E Ink Holdings Inc (Taiwan)
Sony Corporation (Japan)
Pervasive Display Inc (Taiwan)
Samsung Display Co, Ltd (South Korea)
LG Display Co Ltd. (South Korea)
Plastic Logic GmbH (Germany)
Cambrios Technologies Corporation (US)
Bridgestone Corporation (Japan)
Visionect (Slovenia)
CLEARink Displays (US)
Global E-Paper Display Market Scope and Market Size
E-paper display market is segmented on the basis of product, type, technology and end user. The growth among segments helps you analyse niche pockets of growth and strategies to approach the market and determine your core application areas and the difference in your target markets. • E-paper display market on the basis of product has been segmented as e-readers, mobile devices, smart cards, poster & signage, auxiliary displays and electronic shelf label, and wearables. • Based on technology, e-paper display market has been segmented into electrophoretic display, electrowetting display, cholesteric display, interferometric modular display, and others. • On the basis of type, e-paper display market has been segmented into flat EPDs, curved EPDs, flexible EPDs, and foldable EPDs • E-paper display has also been segmented on the basis of end user into automotive, consumer electronics, retail, healthcare and media & entertainment.
E-PAPER DISPLAYS EXPLAINED We so often eulogise about the limitless possibilities e-paper displays enable and the exciting opportunities the technology creates for market growth, differentiation and competitive advantage — but for the uninitiated reader, we thought it might be helpful in this blog to take a step back and look at how e-paper works.
E-paper goes by many names and spellings — electronic paper, ePaper, electronic ink, e ink, electrophoretic displays, EPD — but all these terms effectively describe the same thing: an electrically-charged surface that replicates the look and experience of ink on paper.
Instead of a traditional display that uses backlighting to illuminate pixels, e-paper is based on the science of “electrophoresis” — i.e. the movement of electrically charged molecules in an electric field.
In every e-paper display there are millions of tiny microcapsules containing (negatively charged) black and (positively charged) white pigments suspended in a clear fluid. This encapsulated ‘ink’ is then printed onto a plastic film and laminated on to a layer of circuitry, or — to be even more specific — a transistor matrix layer. The circuitry forms a pattern of pixels that is then controlled by a display driver (EPD controller).
When a negative electric field is applied to the ‘ink’, the white particles move to the top of the capsule making the surface appear white at that specific spot. Reverse this process and the black particles appear at the top making the surface of the capsule appear dark. The technology can also work in colour in just the same way but using a combination of different colour pigments and electric charges, or just by adding a colour filter on top of the display.
The way e-paper works differs from traditional displays in two key ways:
E-paper screens are reflective — light from the environment is reflected from the surface of the e-paper display towards the user’s eyes, just like with traditional paper. This gives e-paper a wide viewing angle that is readable in direct sunlight. E-paper screens are bi-stable — unlike conventional backlit flat panel LCD displays, which refresh about 30 times per second and require a constant power supply to maintain content, e-paper displays will hold a static image ‘forever’, even without electricity. E-paper only consumes power when the content on it changes – for example if an e-paper shelf label in a supermarket is updated with a new price. The rest of the time the display will simply show the content you want it to, where it doesn’t draw any power until the next update.
Making e-paper flexible
Here at Plastic Logic Germany, we took e-paper one stage further and successfully industrialised a process to create glass-free backplanes, which represents the transistor matrix layer mentioned above. We are the first company worldwide able to manufacture transistor arrays on plastic. Instead of using traditional silicon transistors, our active-matrix backplane consists of organic thin film transistors (OTFTs) made from the same plastic used to for cola bottles (PET). This means we can couple a flexible backplane with a flexible display medium, such as flexible OLED or flexible electrophoretic layer, to create a fully flexible display with limitless possibilities. In addition to the flexibility, our glass-free electrophoretic displays also more robust, shatterproof and lightweight compared to glass-based displays.
If you want to know more about flexible plastic e-paper display technology’s suitability for a given use case and to get some inspiration via the applications which are already successfully showcasing the opportunities and rewards achievable through flexible e-paper innovation check out our latest flexible e-paper whitepaper.
By Plastic Logic
Flexible Displays
Flat
Curved
Foldable
Rollable
Printable
Without Glass
On Plastic
Electrowetting Displays (EWD)
LiquaVista
Etulipa
ADT
CMY Colors vs RGB Colors
Source: Current commercialization status of electrowetting-on-dielectric (EWOD) digital microfluidics
The emergence of electrowetting-on-dielectric (EWOD) in the early 2000s made the once-obscure electrowetting phenomenon practical and led to numerous activities over the last two decades. As an eloquent microscale liquid handling technology that gave birth to digital microfluidics, EWOD has served as the basis for many commercial products over two major application areas: optical, such as liquid lenses and reflective displays, and biomedical, such as DNA library preparation and molecular diagnostics. A number of research or start-up companies (e.g., Phillips Research, Varioptic, Liquavista, and Advanced Liquid Logic) led the early commercialization efforts and eventually attracted major companies from various industry sectors (e.g., Corning, Amazon, and Illumina). Although not all of the pioneering products became an instant success, the persistent growth of liquid lenses and the recent FDA approvals of biomedical analyzers proved that EWOD is a powerful tool that deserves a wider recognition and more aggressive exploration. This review presents the history around major EWOD products that hit the market to show their winding paths to commercialization and summarizes the current state of product development to peek into the future. In providing the readers with a big picture of commercializing EWOD and digital microfluidics technology, our goal is to inspire further research exploration and new entrepreneurial adventures.
Source: Stretchable and reflective displays: materials, technologies and strategies
Liquavista technology was acquired by Samsung and then later was sold to Amazon. Amazon has put it on shelves.
Source: Biological versus electronic adaptive coloration: how can one inform the other?
Electrowetting Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
ElectroFluidic Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Source: IllumiPaper: Illuminated Interactive Paper
In general, display technologies can be classified in pixel-addressable high-resolution displays (e.g. OLED, e-paper) and in segment displays, which highlight predefined shapes based on electrochromism (EC) [2], thermochromism (TE) [31] or electroluminescence (EL) [1, 46]. Although advanced display types, such as e-paper, have promising properties (e.g., preserving content without battery), we focus on lightweight, low-current-consuming, segment-based EL and EC displays, which are robust, inexpensive and easy to integrate with pen interaction.
Source: Stretchable and reflective displays: materials, technologies and strategies
Electrochromic Displays
Source: Review Paper: A critical review of the present and future prospects for electronic paper
This technology was formerly by Swedish company Rdot, which was co-founded by Karlsson, and bought by Ynvisible Interactive of Vancouver. The company has prototyping capabilities in Linköping, Sweden and in Almada, Portugal, roll-to-roll production in Sweden, and R&D in Freiburg, Germany.
Printed Electrochromics boldly goes where no display has gone before
Fig.1 Example use case for printed electrochromics: a shock detector smart label with an interactive printed interface.
Expanding Need for Simple Electronic Display Functionality
Rapid advances in the miniaturization and reduction of costs in computing, electronic sensing, and communications have allowed the integration of “smart” electronic functionality into almost everything. ”Intelligence” is now embedded into a wide range of everyday objects, and spread throughout our working and living environments. Much of this intelligence, data collection and transfer is hidden from the human senses, requiring little or no human involvement. But as the number of human daily touch points and interactions with smart devices grows, so too does the importance of user experience design and the role of displays.
Conventional electronic displays cannot be economically and sustainably applied into all smart objects and environments and can often times be functionality overkill for the simple display requirements of many everyday objects. Also, user experiences built around the need for extensive use of separate reading devices, e.g. RFID or Bluetooth readers in smart phones, can be increasingly challenging especially with the high number of distractions and strong competition for attention on mobile screens. Further with a doubling of screen time over the past four years among certain user demographics, there is also a growing sense of screen fatigue leading to people “detoxing” from light emitting screens while still valuing user interfaces that are useful yet unobtrusive.
“As technology becomes ubiquitous, it also becomes invisible.“ – Kevin Kelly, Wired magazine Founding Executive Editor
When technology becomes ubiquitous, it needs to seamlessly blend into the product and our surroundings. The user experience should be effortless. As the “computing” or intelligence blends into smart objects and environments, also the displays need to become more practical: i) eliminating the need for recharging or replacing of batteries, ii) eliminating the amount of effort to access information, and iii) be inexpensive for intended purpose.
Printed Electrochromics Brings Everyday Printable Objects and Surfaces to Life
Electrochromic devices (ECD) are electrochemical cells where color changes occur upon electrochemical reactions of two or more redox active electrochromic materials electrically connected by an external circuit and physically separated by an ionic conducting layer (electrolyte layer). Electrochromic materials and devices can be controlled to change their color and opacity by the application of electrical stimuli. ECDs are a non-light emitting reflective technology. Materials for ECD manufacture can be taken into the form of printable inks and the manufacturing processes made compatible with standard graphic printing and converting processes. The resulting device can be made thin, flexible, transparent, robust, and ultra low-power. As ECDs can be produced into a wide range of different shapes and sizes, they offer a wide range of advantages for product design and integration.
Fig.2 Electrochromic devices can be printed in sheet-to-sheet or roll-to-roll. Ynvisible Production R2R line in Linköping, Sweden.
R&D toward printed electrochromics began in the 1990s. In recent years, with strong advances in printed electronic and hybrid electronic systems, developments of ECDs have made strong technical progress into mass-manufacturability. Electrochromic displays and visual indicators are now entering markets that are considered “blue ocean” from the perspective of the electronic display industry. In these market spaces conventional printed products and surfaces now meet electronics. The over 800 billion USD per year printing industry, and particularly the industrial printing sub-sector, are welcoming printed electronic systems with high level of interest.
Things Alive
Today Ynvisible Interactive Inc. (“Ynvisible”) is leading the charge to bring printed electrochromics into market. Ynvisible was established with the vision to bring everyday objects and surfaces to life benefitting people in a smart and connected world. The company’s mission is to provide practical human interfaces to smart everyday objects and ambient intelligence. After early explorations into different chromogenic systems the company focused on developing electrochromics into a mass producible, ultra-low power consuming visual interface technology. The company now develops and commercializes different printed electrochromic systems on film materials. By combining other printed electronic components and microelectronics into the electrochromic system, the company designs and produces interactive graphic solutions for everyday smart objects and surfaces.
Ynvisible aims to be the leading supplier of design tools, inks and quality control systems for the design and production of interactive printed graphics based on printed electrochromics and other printed electronics technologies. The company is building its technology and products platform under the ynvisible™ brand (ynvisible is a registered trademark in certain countries and territories).
Fig. 3 Electrochromic displays on a temperature label provide clear visual indication and are easy to implement – user friendly and available in high volumes.
Ynvisible’s primary focus is on applications in retail and logistics (where ECDs are printed onto RFID tags and RF-based smart labels), premium consumer brand products, and healthcare and wellness (in particular medical and diagnostic devices). Today the company offers a full services package to help product developers and designers get started with printed electrochromics. Ynvisible’s design, prototyping, customer training and sheet-to-sheet production services are based in Almada, Portugal. The company’s inks development and R&D services are based in Freiburg, Germany. In Linköping, Sweden the company operates a roll-to-roll production facility with extensive printing, converting, and quality control system capabilities. In addition to high volume ECD printing, the high capacity production line is utilized for printing of other printed electronic components and systems. Ynvisible sells printed electronics production upscaling services to other product owning companies.
Ynvisible Interactive Inc. is a publicly traded company, listed in the Toronto Stock Exchange Venture list [TSXV:YNV], the OTC Markets [OTCQB:YNVYF] and the Frankfurt Stock Exchange [FRA:1XNA]
Getting Started With Printed Electrochromics
To learn more about Printed Electrochromics, Ynvisible is hosting a free webinar on Apr 2, 2020 12:00-1:00 PM in Eastern Time (US and Canada). The one hour webinar includes speakers from the Georgia Institute of Technology, NXN-IP and the University of Lapland. To register see: https://www.ynvisible.com/events
Fig.4 Printed paper label with printed NFC antenna and printed electrochromic display on the same substrate. A collaboration between Arjowiggins and Ynvisible.
Worldwide Industry for Electrochromic Materials to 2025 – Impact of COVID-19
Technology launches, acquisitions, and R&D activities are key strategies adopted by players in the electrochromic materials market. In 2019, the market of electrochromic materials has been consolidated by the top ten players accounting for 65.4% of the share. Major players in the electrochromic materials market are Gentex corporation, Saint Gobain, View, Inc., ChromoGenics, AGC, Inc., Changzhou Yapu New Materials Co. Ltd., Magna Glass and Window Company Inc., Econtrol-Glass Gmbh & Co. KG, Nikon Corporation, and Zhuhai Kaivo Optoelectronic Technology Co. Ltd. among others.
Region wise, the market is segmented into North America, Europe, Asia-Pacific, and LAMEA. Europe was the highest revenue contributor in 2019. The presence of leading automotive manufacturers using electrochromic glass is expected to drive the growth of the market. Electrochromic glasses yield better energy savings and comfort and are increasingly used in panoramic roofing in cars. It is already being used in Mercedes-Benz SLK and SL roadsters. Similarly, in 2019, AGP Group, one of the world’s leading glazing manufacturers, opened its automotive glazing plant in Belgium for producing panoramic roofs with electrochromic glass. This trend indicates a growing market for electrochromic glasses in automotive applications.
Smartphone manufacturers are developing phones containing electrochromic glasses to increase their share in the global electrochromic glass market share. For instance, at CES 2020, Chinese phone maker, OnePlus announced Concept One phone that uses electrochromic glasses to hide its rear triple cameras, when not in use. This phone is not expected to be mass produced but it opens a new end-use for electrochromic glass in the smartphone market, which is largely dominated by countries such as China, India, and Japan. Further, China is one of the world’s largest smartphone makers.
The major electrochromic glass manufacturers analyzed in this report include AGC Inc., ChromoGenics AB, Compagnie de Saint-Gobain S.A., Hitachi Chemical Co. Ltd., Kinestral Technologies Inc., Pleotint LLC, Polytronix Inc., Research Frontiers Inc., Smartglass International Ltd., and View Inc. To stay competitive, these market players are adopting different strategies such as product launch, partnership, merger, and acquisition. For instance, on December 2017, AGC, Kinestral Technologies Inc. and G-Tech Optoelectronics Corp. announced a joint venture that will sell, distribute, and service Halio smart glasses to the global market. The new ventures are Halio North America, Halio International, and Halio China. The joint venture helped AGC to increase its market revenue.
Multilayered ECD by Ricoh using CMY Colors
Source: Multi-Layered Electrochromic Display
Reflective Displays based on Conventional LCD
Reflective LCD
Japan Display Inc – MIP Reflective LCD
Samsung – SR (Super Reflectance) LCD Technology
Types of Reflective LCDs
Direct View
Projection
Japan Display Inc.
Japan Display Inc (JDI) is an LCD technology joint venture by Sony, Toshiba, and Hitachi since 2012.
Memory-in-pixel (MIP) Reflective Color LCD
Japan Display Inc. (JDI), a leading global supplier of small- and medium-sized displays, has announced the start of sales of a standard line-up of memory-in-pixel (MIP) reflective-type color LCD modules for wristwatch-type wearable devices which realize ultra-low power consumption. Power consumption of these reflective-type LCD modules is less than 0.5%*1 that of transparent-type LCD modules.
Source: Japan Display Inc.
Source: JAPAN DISPLAY SHOWS LOW-POWER REFLECTIVE LCD THAT DOES COLOR, VIDEO
Emerging Display Technologies
MIP Reflective Color LCD for Ultra Low Power Consumption
Organic Electro-Luminescent (EL) display for High Contrast and Thin Structure
Transparent Display
Light field display (LFD) for 3D Definition (Holographic)
Micro LED display
Hybrid OLED and Reflective LCD
Transflective LCD Displays
Transflective LCDs combine elements of both transmissive and reflective characteristics. Ambient light passes through the LCD and hits the semi-reflective layer. Most of the light is then reflected back through the LCD. However some of the light will not be reflected and will be lost. Alternately a backlight can be used to provide the light needed to illuminate the LCD if ambient light is low. Light from the backlight passes through a semi-reflective layer and illuminates the LCD. However as with ambient lighting some of the light does not penetrate the semi-reflective layer and is lost.
Depends both on Transmission and Reflection.
Types of Transflective LCDs
Source: Fundamentals of Liquid Crystal Devices
Based on the light modulation mechanisms, transflective LCDs can be classified into four categories:
Color-reflective LCD based on cholesteric liquid crystals
Kent Displays Inc.
Cholesteric liquid crystal (CLC)
Kent State University/Deng-Ke Yang
Cholesteric liquid crystals (hereafter Ch LCs) are self-assembled systems consisting of elongated chiral organic molecules. They possess a helical structure where the local average direction of the molecules twists spatially around an orthogonal helical axis. Their refractive index varies periodically, and thus exhibits a Bragg reflection band centered at the wavelength λ = [(ne + no)/2]P and with the bandwidth Δλ = (ne – no)P, where ne and no are the extraordinary and ordinary refractive indices of the LC, respectively, and P is the helical pitch. They can be used to make reflective displays which do not need polarizers and have high reflectance.
Dr. J. William Doane is a world renowned expert in the field of liquid crystal materials and devices. Together with William Manning he co-founded Kent Displays, Inc. in 1993 and the company is now famous for its Cholesteric LCD based Boogie Board writing tablets that use Dr Doane’s inventions. Dr. Doane, was the director of the world-renowned Liquid Crystal Institute at Kent State University from 1983-1996 and led the effort during that time to establish the National Science Foundation Center for Advanced Liquid Crystalline Optical Materials (ALCOM). As an active member of the international science community, he has held visiting appointments and maintained cooperative research programs in several countries. Dr. Doane was instrumental in formalizing the International Liquid Crystal Society and served as the organization’s first treasurer from 1990-1996. Dr. Doane was named a Fellow of the American Physical Society in 1982 and retired from the Kent State University in 1996 after a 31-year teaching and administrative career. Dr. Doane received the first ever presentation of the Slottow-Owaki Prize for Display Education, by SID.
Source: Bistable Liquid Crystal Displays
Cholesteric LCD Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Reflective Displays Using Structural Colors
Interferometric Modulator Display (IMOD)
Photonic crystal displays (P-Ink)
Plasmonic Structural Colors Displays
Source: : E SKIN Displays
A wide range of reflective displays have recently been developed, and Fig. XX compares several of the most prominent. Much like color generation in animals, reflective displays can be separated into the same two main categories; pigmentation and structural color. Under the tent of pigmentation are products such as e-ink based “E-readers”, and color filter based liquid crystal displays. E-readers use the translocation of 3 charged pigmented beads, and as such, require seconds to switch between images. Due to the macroscopic size of each pixel, resolution and color reproduction are also limited. Reflective liquid crystal displays are much quicker, taking only milliseconds to switch states, but are limited in brightness as polarizers immediately halve the amplitude of the reflected light. Many structural color based displays are currently in development and have only recently entered the market. One such device is an interferometric modulator produced by Qualcomm where within each pixel, a cavity is formed between a Bragg stack and a MEMs mirror. By controlling the cavity spacing, the reflected light experiences either constructive or destructive interference resulting in a bright color or dark state. While producing the signature bright vivid colors of Bragg reflection, the device is inherently angle sensitive and limited to rigid substrates. Another emerging structural color based device uses a photonic crystal made from silica spheres submerged within an electro-active polymer, and is branded Photonic Ink (P-ink). The polymer stretches as a field is applied, increasing the period of the photonic crystal and therefore the wavelength of reflected light. Though the colors are vivid and tunable, the response time of the polymer is tens of seconds, making video impossible. Cholesteric and blue phase LC displays behave in a similar manner. Helixes of LC form periodic nanostructures which produce Bragg reflections at desired wavelengths. The LC structures can be switched through an external field thereby producing dark and light states. While producing vivid color, these devices are limited in brightness as the helical structures only reflect circular light of the same handedness of the LC. By assessing current technologies we determine there is much to understand and develop in order to truly mimic color generation in nature. A fast response, angle independent LC-metasurface based display which can actively shift the color of its pixels from RGB to black holds the promise for development of truly thin-film flexible displays.
Interferometric Modulator Display (IMOD)
(Structural Interference Colors)
Qualcomm Mirasol
MEMS Micro-Electro-Mechanical-Systems
MEMS Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Source: Biological versus electronic adaptive coloration: how can one inform the other?
Photonic crystal displays (P-Ink)
(Structural Colors)
Opalux
Nanobrick
Photonic Crystals Display
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Source: Review Paper: A critical review of the present and future prospects for electronic paper
Photonic crystals refer to a class of structures that consist of periodic nanostructures with a spacing that interacts with the propagation of electromagnetic waves. Based on the spacing of the nanostructures, certain energy bands are allowed or forbidden for propagation, which can lead to the selective transmission or reflection of light. Dynamically changing the spacing of the nanostructures can change these propagation properties, leading to the modulation necessary to build a display. The term photonic crystal in displays is usually associated with three-dimensional periodic lattices, and the previously described MEMS interferometry (Sec. 1) is a special one-dimensional case of a photonic crystal.
Currently, there are two different photonic-crystal- based modulation approaches in development. Opalux is fabricating electrically color-tunable photonic crystals by embedding the lattices of 200-nm-diameter silica beads within an expandable electroactive polymer, which they call Photonic Ink or P-Ink.142 Opalux has demonstrated bistable P-Ink with a reflectance >50% and switching speed ~0.1 sec.143 The company has not described the viewing-angle dependence of their technology; photonic-crystal structures that possess highly regular crystal structures can show sharp dependences on illumination and viewing angles.
The company Nanobrick is developing systems that control the inter-particle distance of SiOx-encapsulated metal nanoparticles (20–30 wt.%) in electrophoretic colloidal suspension.144 These photonic crystal structures respond to signals of a few volts, shifting the reflected color through a continuous range as the average spacing changes. This enables full-spectrum tunability using a single electro-optic layer without requiring individual primary-color subpixels to generate color. It appears that the electrophoresis tends to randomize the structure somewhat, leading to reasonably wide viewing angles. Nanobrick has demonstrated a Color Tunable Photonic Crystal Display (CPD) with angle-independent optical responses (0–40°) using quasi-amorphous photonic pixels with response time <50 msec.145
While single-layer color tuning is a unique capability of the photonic-crystal approaches, the technology focus thus far has been limited to unit pixel or simple segments and still needs refinement in terms of the white state, reflectance vs. illumination condition, and demonstration with matrix addressing. Even though, in theory, colors such as red can be displayed at all pixels, white is still challenged because white will likely require additive display of side-by-side RGB pixels [similar to Fig. 2(b)]. Additionally, since pixels currently do not possess inherent gray scale, that means gray scale at the display level will require halftoning approaches.
Source: Stretchable and reflective displays: materials, technologies and strategies
Source: Stretchable and reflective displays: materials, technologies and strategies
Source: P-Ink and Elast-Ink from lab to market
Plasmonic Structural Colors Displays
Source: Dynamic plasmonic color generation enabled by functional materials
Structural colors, well known from coloration in nature (7), can overcome these limitations. Different from dyes and pigments, structural colors are generated by the interaction of light with micro- and nanostructures. Vibrant colors can be produced with the same materials (e.g., metals or dielectrics) by changing the geometries, dimensions, or arrangements of the structures through the fabrication process or even after fabrication (8). Compared to pigment or dye-based coloration, colors created in this case are much brighter due to their inherently high scattering/absorption efficiencies. As a result, thin layers, or more precisely tiny volumes, are sufficient for brilliant coloration. The benefit of these small coloration volumes is obvious. Ultrahigh-resolution images composed of subwavelength pixels with sizes down to the smallest coloring unit, e.g., a single micro- or nanostructure, can be printed (9). In addition, structural colors do not fade over time but provide a basically everlasting coloration due to the stability of the coloring structures. These appealing advantages have attracted great interest and stimulated intensive research on various structural coloration schemes based on metal nanostructures, dielectric metasurfaces, photonic crystals, and Fabry-Perot (FP) resonances (6, 10–17).
Source: Plasmonic Color Makes a Comeback
Plasmonic color is a subset of structural color, which is color resulting when the micro- or nanostructure of a material causes light scattering and interference. One form of structural color is the iridescent blue of the Morpho butterfly’s wings, whose scales have branched nanostructures that scatter light in complex ways. In plasmonic color, the color arises from light absorption and scattering off of the nanoparticles themselves. As with other forms of structural color, size, shape, and patterning create the color rather than chemical composition.
Source: Plasmonic Color Makes a Comeback
The Naval Research Laboratory’s Fontana has a different approach to making dynamic plasmonic displays: using self- assembled colloidal gold nanorods suspended in toluene. By placing an electric field across the suspension, the nanorods align in the direction of the applied field, producing intense plasmonic color, Fontana explains. The system is fast; it can switch at least 1,000 times as quickly as a conventional liquid-crystal pixel, potentially cutting down on motion blur, which is a problem with LCD displays.
Source: Plasmonic Color Makes a Comeback
In current commercial displays, each pixel is actually made of a red, green, and blue subpixel. Different amounts of light from each subpixel mix to create the perception of any color desired. One ambition for those developing plasmonic color systems is to flip one pixel between red, green, and blue rather than needing three separate subpixels that would require less space, allowing for much smaller pixels and higher definition screens. A system that can do just that has been created in the lab of Jeremy Baumberg at the University of Cambridge. It uses gold nanoparticles coated in the conducting polymer polyaniline and sprayed onto a flexible mirrored surface. The mirrored surface amplifies the plasmonic resonance, resulting in a more intense, uniform color with no viewing-angle dependence.
The color of each pixel is tuned by the reversible oxidation and reduction of the polymer, which changes the polymer’s refractive index and shifts the system’s plasmonic resonance. Each nanoparticle can theoretically be tuned independently, providing a potential spatial resolution of less than 100 nm. So far, the researchers have created pixels that switch only between red and green, but they are working on blue. Silver or aluminum particles could potentially show blue color, but “there is always a trade- off, as silver and aluminum materials are chemically [more] unstable [than gold],” says Hyeon-Ho Jeong, who formerly worked as a postdoc with Baumberg at Cambridge and is now at Gwangju Institute of Science and Technology.
Key Terms
Plasmonic Metasurfaces
Plasmonic Nanostructures
Conjugated Polymers
Plasmons
Metallic Nanostructures
Functional Materials
Plasmonic Resonances
Liquid Crystals plus plasmonic nanostructure
Metal surface plasmonics
Nanowire waveguides
Meta-materials
Quantum dots (QDs)
Nano Hole Array NHA
Dielectric Metasurfaces
Tunable Color Filters
Metal-Insulator-Metal Resonators (MIM)
Sub Wavelength Grating SWG
Subwavelength metal–insulator–metal stack arrays
Nanowire Color Filter
Metasurface Color Filter
Quantum Dot Color Filter
Plasmonic hole array color filters
Debashis Chanda, a nanophotonics scientist at the University of Central Florida
E-skin Displays, in 2017
Source: Plasmonic Metasurfaces with Conjugated Polymers for Flexible Electronic Paper in Color
Transmissive Displays
Emmisive Displays
LED
True QLED
OLED
AM-OLED
Mini-LED
Micro LED
All QLED panels are made by Samsung. QLED really is LCD panel It requires back lighting.
All OLED panels are made by LG Displays.
Please see my post on LCD and LED displays.
Transmitive Displays
LCD
AM LCD
PMLCD
Please see my post on LCD and LED displays.
Transmissive liquid crystal displays (LCDs) have been widely used in laptop computers, desktop monitors, and high-definition televisions (HDTVs).
TFT- AMLCD
Source: Review of Display Technologies Focusing on Power Consumption
In AMLCD, a switch is placed at each pixel which decouples the pixel-selection function. Thin Film Transistor (TFT), the main technology of the AMLCD subgroup, can also be divided regarding the material used for its elaboration, into amorphous silicon (a-Si), continuous grain silicon (CGS) and low temperature polycrystalline silicon (LTPS TFT). A new approach is the indium-gallium-zinc- oxide (IGZO) technology developed by Sharp.
Another issue to take into account is the liquid crystal alignment mode, where Twisted Nematic (TN) and Super-Twisted Nematic (STN) types are the simplest and least expensive, but offering a poor viewing angle (of approx. 45 degrees). Vertical Alignment (VA) technology generally appears under various trade names (ASV by Sharp, PVA by Samsung, etc.) and tries to improve the viewing angle of the device (for instance, Ampire VA device offers 160 degrees versus the 45 of the TN device by AUO). In-plane switching (IPS TFT), as the Hitachi module from the table shows, also has a better viewing angle than TN and the color and contrast is also improved.
B. J. Feenstra1, R. A. Hayes, R. van Dijk, R. G. H. Boom, M. M. H. Wagemans, I. G. J. Camps, A. Gi- raldo and B. v.d. Heijden Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA, Eindhoven, The Netherlands
Dyed Polymeric Microparticles for Colour Rendering in Electrophoretic Displays
Mark Goulding, Louise Farrand, Ashley Smith, Nils Greinert, Henry Wilson, Claire Topping, Roger Kemp, Emily Markham, Mark James, Johannes Canisius, Dan Walker Merck Chemicals Ltd., Advanced Technologies, Chilworth Technical Centre, University Parkway, Southampton, Hampshire, SO16 7QD, UK
Richard Vidal, Sihame Khoukh
Merck Chimie, Center Production ESTAPOR, Zone Industrielle, Rue du Moulin de la Canne, 45 300 Pithiviers, France.
Seung-Eun Lee, Hee-Kyu Lee
Merck Advanced Technologies, Poseung Technical Center, 1173-2 Wonjyung-ri, Poseung-myun, Pyungtaek-si, Kyungki-do, Korea
E Ink Holdings, electronic ink technology, and Fujitsu Semiconductor have developed a reference design board for battery-less ePaper tags using E Ink’s ePaper and Fujitsu’s UHF (Ultra High Frequency) band.
Shin-Tson Wu, Hughes Research Laboratories Deng-ke Yang, Kent State University
The evolution of portable communications applications has been facilitated largely by the development of reflective LCD technology. Offering a unique insight into state-of-the art display technologies, Reflective Liquid Crystal Displays covers the basic operations principles, exemplary device structures and fundamental material properties of device components.
Featuring:
Direct-view, projection and micro (virtual projection) reflective displays in the context of multi-media projectors, mobile internet and personal entertainment displays.
Optimization of critical display attributes: fast response time, low voltage operation and wide angle viewing.
Description of the basic properties of liquid crystal materials and their incorporation into configurations for transmissive and reflective applications.
Examination of the various operations modes enabling the reader to select the appropriate display type to meet a variety of needs.
Overview and comparison of the complete range of reflective display technologies, and reflective LCD effects.
Product Demographics Author: Shin-Tson Wu, Deng-Ke Yang Publisher: John Wiley & Sons, Ltd Date of Publication: 04/01/2001 ISBN Number: 0-471-49611-1 Format: Hardback Pages: 352
Reflective liquid crystal display with fast response time and wide viewing angle
Rollable and transparent subpixelated electrochromic displays using deformable nanowire electrodes with improved electrochemical and mechanical stability
Laboratory of Organic Electronics, Department of Science and Technology, Linkoping University, SE-601 74 Norrkoping, Sweden2RISE Acreo, ICT Department, Printed Electronics, Research Institutes of Sweden, Acreo, 601 17 Norrkoping, Sweden
Electrokinetic pixels with biprimary inks for color displays and color-temperature-tunable smart windows
S. MUKHERJEE,1 W. L. HSIEH,3 N. SMITH,2 M. GOULDING,2 AND J. HEIKENFELD1,*
1Department of Electrical Engineering and Computing Systems, University of Cincinnati, Cincinnati, Ohio 45221, USA 2Merck Chemicals Ltd., Chilworth Technical Centre, Southampton, Hampshire SO16 7QD, UK 3Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan *Corresponding author: heikenjc@ucmail.uc.edu
Received 11 March 2015; revised 5 May 2015; accepted 11 May 2015; posted 18 May 2015 (Doc. ID 235176); published 10 June 2015
Novel organic multi-color electrochromic device for e-paper application
Authors: Kobayashi, Norihisa; Yukikawa, Masahiro; Liang, Zhuang; Nakamura, Kazuki Source: NIP & Digital Fabrication Conference, Volume 2017, Number 1, November 2017, pp. 111-114(4) Publisher: Society for Imaging Science and Technology
Electrochromic materials and devices: present and future
Prakash R. Somani a,∗, S. Radhakrishnan b
a Photonics and Advanced Materials Laboratory, Centre for Materials for Electronics Technology (C-MET), Panchawati, Off Pashan Road, Pune 411008, India b National Chemical Laboratory (NCL), Polymer Science and Chemical Engineering, Pune 411008, India
Received 17 May 2001; received in revised form 10 September 2001; accepted 26 September 2001
TiO2 Nanostructured Films for Electrochromic Paper Based-Devices
by Daniela Nunes , Tomas Freire, Andrea Barranger 1, João Vieira 1, Mariana Matias 1, Sonia Pereira 1, Ana Pimentel 1, Neusmar J. A. Cordeiro 1,2, Elvira Fortunato 1 and Rodrigo Martins 1,
Shin-Tson Wu, Hughes Research Laboratories Deng-ke Yang, Kent State University
The evolution of portable communications applications has been facilitated largely by the development of reflective LCD technology. Offering a unique insight into state-of-the art display technologies, Reflective Liquid Crystal Displays covers the basic operations principles, exemplary device structures and fundamental material properties of device components.
Display engineers, scientists and technicians active in the field will welcome this unique resource, as will developers of a wide range of systems and applications. Graduate students and researchers will appreciated the introduction and technical insight into this exciting technology.
Featuring:
Direct-view, projection and micro (virtual projection) reflective displays in the context of multi-media projectors, mobile internet and personal entertainment displays.
Optimization of critical display attributes: fast response time, low voltage operation and wide angle viewing.
Description of the basic properties of liquid crystal materials and their incorporation into configurations for transmissive and reflective applications.
Examination of the various operations modes enabling the reader to select the appropriate display type to meet a variety of needs.
Overview and comparison of the complete range of reflective display technologies, and reflective LCD effects.
Author: Shin-Tson Wu, Deng-Ke Yang Publisher: John Wiley & Sons, Ltd Date of Publication: 04/01/2001 ISBN Number: 0-471-49611-1 Format: Hardback Pages: 352
Electrofluidic Displays: Multi-stability and Display Technology Progress
Kenneth A. Dean, Kaichang Zhou, Steve Smith, Brian Brollier, Hari Atkuri and John Rudolph Gamma Dynamics, Cincinnati, OH 45229, U.S.A.
Shu Yang, Stephanie Chevalliot, Eric Kreit, and Jason Heikenfeld
Novel Devices Lab, University of Cincinnati, Cincinnati, OH 45221, U.S.A.
Photonic-crystal full-colour displays
́ ANDRE C. ARSENAULT1,2*, DANIEL P. PUZZO1,2, IAN MANNERS3* AND GEOFFREY A. OZIN1*
1Department of Chemistry, University of Toronto, 80 St George Street, Toronto M5S 3H6, Canada 2Opalux Incorporated, 80 St George Street, Toronto M5S 3H6, Canada 3School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK *e-mail: andre.arsenault@opalux.com; Ian.Manners@bristol.ac.uk; gozin@chem.utoronto.ca
nature photonics | VOL 1 | AUGUST 2007 | www.nature.com/naturephotonics
1Institute of Lighting and Energy Photonics, National Chiao Tung University, Guiren Dist, Tainan 71150, Taiwan 2Institute of Photonic System, National Chiao Tung University, Guiren Dist., Tainan 711, Taiwan 3Faculty of Physics, Adam Mickiewicz University in Poznan, Poland
4Institute of Imaging and Biomedical Photonics, National Chiao Tung University, Guiren Dist., Tainan 71150, Taiwan
Plasmonic Color Filters for CMOS Image Sensor Applications
Sozo Yokogawa,†,‡,§ Stanley P. Burgos,†,§ and Harry A. Atwater*,† †Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States
‡Sony Corporation, Atsugi Tec. 4-14-1 Asahi-cho, Atsugi, Kanagawa, 243-0014, Japan
DANIEL FRANKLIN B.S. Missouri University of Science and Technology, 2011
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the College of Sciences at the University of Central Florida Orlando, Florida
LCD (Liquid Crystal Display) is a type of flat panel display which uses liquid crystals in its primary form of operation. LEDs have a large and varying set of use cases for consumers and businesses, as they can be commonly found in smartphones, televisions, computer monitors and instrument panels.
LCDs were a big leap in terms of the technology they replaced, which include light-emitting diode (LED) and gas-plasma displays. LCDs allowed displays to be much thinner than cathode ray tube (CRT) technology. LCDs consume much less power than LED and gas-display displays because they work on the principle of blocking light rather than emitting it. Where an LED emits light, the liquid crystals in an LCD produces an image using a backlight.
As LCDs have replaced older display technologies, LCDs have begun being replaced by new display technologies such as OLEDs.
How LCDs work
A display is made up of millions of pixels. The quality of a display commonly refers to the number of pixels; for example, a 4K display is made up of 3840 x2160 or 4096×2160 pixels. A pixel is made up of three subpixels; a red, blue and green—commonly called RGB. When the subpixels in a pixel change color combinations, a different color can be produced. With all the pixels on a display working together, the display can make millions of different colors. When the pixels are rapidly switched on and off, a picture is created.
The way a pixel is controlled is different in each type of display; CRT, LED, LCD and newer types of displays all control pixels differently. In short, LCDs are lit by a backlight, and pixels are switched on and off electronically while using liquid crystals to rotate polarized light. A polarizing glass filter is placed in front and behind all the pixels, the front filter is placed at 90 degrees. In between both filters are the liquid crystals, which can be electronically switched on and off.
LCDs are made with either a passive matrix or an active matrix display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, improving the screen refresh time.
Some passive matrix LCD’s have dual scanning, meaning that they scan the grid twice with current in the same time that it took for one scan in the original technology. However, active matrix is still a superior technology out of the two.
Types of LCDs
Types of LCDs include:
Twisted Nematic (TN)- which are inexpensive while having high response times. However, TN displays have low contrast ratios, viewing angles and color contrasts.
In Panel Switching displays (IPS Panels)- which boast much better contrast ratios, viewing angles and color contrast when compared to TN LCDs.
Vertical Alignment Panels (VA Panels)- which are seen as a medium quality between TN and IPS displays.
Advanced Fringe Field Switching (AFFS)- which is a top performer compared IPS displays in color reproduction range.
LCD vs OLED vs QLED
LCDs are now being outpaced by other display technologies, but are not completely left in the past. Steadily, LCDs have been being replaced by OLEDs, or organic light-emitting diodes.
OLEDs use a single glass or plastic panels, compared to LCDs which use two. Because an OLED does not need a backlight like an LCD, OLED devices such as televisions are typically much thinner, and have much deeper blacks, as each pixel in an OLED display is individually lit. If the display is mostly black in an LCD screen, but only a small portion needs to be lit, the whole back panel is still lit, leading to light leakage on the front of the display. An OLED screen avoids this, along with having better contrast and viewing angles and less power consumption. With a plastic panel, an OLED display can be bent and folded over itself and still operate. This can be seen in smartphones, such as the controversial Galaxy Fold; or in the iPhone X, which will bend the bottom of the display over itself so the display’s ribbon cable can reach in towards the phone, eliminating the need for a bottom bezel.
However, OLED displays tend to be more expensive and can suffer from burn-in, as plasma-based displays do.
QLED stands for quantum light-emitting diode and quantum dot LED. QLED displays were developed by Samsung and can be found in newer televisions. QLEDs work most similarly to LCDs, and can still be considered as a type of LCD. QLEDs add a layer of quantum dot film to an LCD, which increases the color and brightness dramatically compared to other LCDs. The quantum dot film is made up of small crystal semi-conductor particles. The crystal semi-conductor particles can be controlled for their color output.
When deciding between a QLED and an OLED display, QLEDs have much more brightness and aren’t affected by burn-in. However, OLED displays still have a better contrast ratio and deeper blacks than QLEDs.
An LCD or liquid crystal display is a type of flat panel display commonly used in digital devices, for example, digital clocks, appliance displays, and portable computers.
How an LCD Works
Liquid crystals are liquid chemicals whose molecules can be aligned precisely when subjected to electrical fields, much in the way metal shavings line up in the field of a magnet. When properly aligned, the liquid crystals allow light to pass through.
A simple monochrome LCD display has two sheets of polarizing material with a liquid crystal solution sandwiched between them. Electricity is applied to the solution and causes the crystals to align in patterns. Each crystal, therefore, is either opaque or transparent, forming the numbers or text that we can read.
History of Liquid Crystal Displays
In 1888, liquid crystals were first discovered in cholesterol extracted from carrots by Austrian botanist and chemist, Friedrich Reinitzer.
In 1962, RCA researcher Richard Williams generated stripe patterns in a thin layer of liquid crystal material by the application of a voltage. This effect is based on an electrohydrodynamic instability forming what is now called “Williams domains” inside the liquid crystal.
According to the IEEE, “Between 1964 and 1968, at the RCA David Sarnoff Research Center in Princeton, New Jersey, a team of engineers and scientists led by George Heilmeier with Louis Zanoni and Lucian Barton, devised a method for electronic control of light reflected from liquid crystals and demonstrated the first liquid crystal display. Their work launched a global industry that now produces millions of LCDs.”
Heilmeier’s liquid crystal displays used what he called DSM or dynamic scattering method, wherein an electrical charge is applied which rearranges the molecules so that they scatter light.
The DSM design worked poorly and proved to be too power hungry and was replaced by an improved version, which used the twisted nematic field effect of liquid crystals invented by James Fergason in 1969.
James Fergason
Inventor James Fergason holds some of the fundamental patents in liquid crystal displays filed in the early 1970s, including key US patent number 3,731,986 for “Display Devices Utilizing Liquid Crystal Light Modulation”
In 1972, the International Liquid Crystal Company (ILIXCO) owned by James Fergason produced the first modern LCD watch based on James Fergason’s patent.
Liquid Crystals in a Display
Source: Japan Display Inc.
LCD Basics
Liquid crystal
Liquid crystal refers to the intermediate status of a substance between solid (crystal) and liquid. When crystals with a high level of order in molecular sequence are melted, they generally turn liquid, which has fluidity but no such order at all. However, thin bar-shaped organic molecules, when they are melted, keep their order in a molecular direction although they lose it in molecular positions. In the state in which molecules are in a uniform direction, they also have refractive indices, dielectric constants and other physical characteristics similar to those of crystals, depending on their direction, even though they are liquid. This is why they are called liquid crystal. The diagram below shows the structure of 5CB (4-pentyl-4’-Cyanobiphenyl) as an example of liquid crystal molecules.
An example of a liquid crystal molecule
Principle of liquid crystal display
A liquid crystal display (LCD) has liquid crystal material sandwiched between two sheets of glass. Without any voltage applied between transparent electrodes, liquid crystal molecules are aligned in parallel with the glass surface. When voltage is applied, they change their direction and they turn vertical to the glass surface. They vary in optical characteristics, depending on their orientation. Therefore, the quantity of light transmission can be controlled by combining the motion of liquid crystal molecules and the direction of polarization of two polarizing plates attached to the both outer sides of the glass sheets. LCDs utilize these characteristics to display images.
Working principle of an LCD
TFT LCD
An LCD consists of many pixels. A pixel consists of three sub-pixels (Red/Green/Blue, RGB). In the case of Full-HD resolution, which is widely used for smartphones, there are more than six million (1,080 x 1,920 x 3 = 6,220,800) sub-pixels. To activate these millions of sub-pixels a TFT is required in each sub-pixel. TFT is an abbreviation for “Thin Film Transistor”. A TFT is a kind of semiconductor device. It serves as a control valve to provide an appropriate voltage onto liquid crystals for individual sub-pixels. A TFT LCD has a liquid crystal layer between a glass substrate formed with TFTs and transparent pixel electrodes and another glass substrate with a color filter (RGB) and transparent counter electrodes. In addition, polarizers are placed on the outer side of each glass substrate and a backlight source on the back side. A change in voltage applied to liquid crystals changes the transmittance of the panel including the two polarizing plates, and thus changes the quantity of light that passes from the backlight to the front surface of the display. This principle allows the TFT LCD to produce full-color images.
After over 120 years of research in liquid crystals, a large number of liquid crystal phases have been discovered. Liquid crystal phases have a range of different structures, but all have one thing in common: they flow in a similar way to viscous liquids, but show the physical behavior of crystals. Their appearance depends on various criteria, including molecular structure and temperature, as well as their concentration and the solvent.
A crystal can be described using a coordinate system. Each atom of a molecule has its specific position. The structure of a crystal can be reduced to a tiny unit, the primitive cell, which is repeated periodically in all three dimensions. This periodicity describes the long-range order of a crystal. A crystal is a highly ordered system in which the physical properties have different characteristics according to the viewing angle. This is called anisotropy. The properties of a liquid crystal phase are also anisotropic, although the structure can no longer be described in a coordinate system. The periodicity and thus the long-range order are lost. Molecules orient themselves by their neighboring molecules, so that only short-range order can be observed. In contrast, a liquid is a completely disordered system, in which the physical properties are isotropic, i.e. directionally independent. What a liquid crystal phase and a liquid have in common is fluidity.
THERMOTROPIC NEMATIC PHASE
In LCD technology, the thermotropic nematic phase is by far the most significant phase. It is formed from rod-shaped (calamitic) molecules that arrange themselves approximately parallel to each other. These molecules can also form smectic phases, which exist in multiple manifestations. Smectic phases are more ordered than nematic phases: as well as the parallel alignment of the molecules, they also form layers.
As the temperature rises, the order of a system decreases. The temperature at which a liquid crystal phase is converted to the isotropic liquid is called the clearing point. A substance may form one or more liquid crystal phases if the structural conditions allow this. However, the appearance of liquid crystal phases is not necessarily a consequence of the molecular structure.
Source: Liquid Crystalline materials used in LCD display
Back Lighting Unit (BLU) – Source of Unpolarized Light
1 st Polarizing Filter – Input Polarizer
Glass Substrate – backbone
Thin Film Transistor (TFT)
TFT Electrode
Orientation Layer – Thin Film Transistors
RM/additive Polymer layer – orientation of liquid crystal molecules and for fixing “pretilt angle”
RM Polymer Layer
Liquid Crystals
Polarized Light
RM/additive polymer layer
RM Polymer layer
Orientation layer
Electrode
Color filter
Glass Substrate
2 nd Polarizing filter
Materials used in making Displays
Source: Merck KGaA Germany
Liquid Crystals
OLED Materials
Photoresists
Siloxanes
Silozanes
LED Phosphores
Quantum Materials
Reactive Mesogens
Three main components of a LCD Panel are discussed below in some detail.
Backplane Technology
Color Filter
Backlighting
Backplane Technology
What Is An LTPS LCD?
August 10, 2019
Low-temperature polycrystalline silicon (or LTPS) LCD—also called LTPS TFT LCD—is a new-generation technology product derived from polycrystalline silicon materials. Polycrystalline silicon is synthesised at relatively low temperatures (~650°C and lower) as compared to traditional methods (above 900°C).
Standard LCDs found in many consumer electronics, including cellphones, use amorphous silicon as the liquid for the display unit. Recent technology has replaced this with polycrystalline silicon, which has boosted the screen resolution and response time of devices.
Row/column driver electronics are integrated onto the glass substrate. The number of components in an LTPS LCD module can be reduced by 40 per cent, while the connection part can be reduced by 95 per cent. The LTPS display screen is better in terms of energy consumption and durability, too.
LTPS LCDs are increasingly becoming popular these days. These have a high potential for large-scale production of electronic devices such as flat-panel LCD displays or image sensors.
Amorphous silicon lacks crystal structure, whereas polycrystalline silicon consists of various crystallites or grains, each having an organised lattice (Fig. 1).
Fig. 1: Amorphous silicon versus polycrystalline silicon (Credit: Wikipedia)
Advantages of an LTPS LCD display are:
Dynamic and rich colours
Fast response and less reflective
High picture resolution
Some of its disadvantages are:
Deteriorates faster than other LCDs
High cost
Display technology explained: A-Si, LTPS, amorphous IGZO, and beyond
LCD or AMOLED, 1080p vs 2K? There are plenty of contentious topics when it comes to smartphone displays, which all have an impact on the day to day usage of our smartphones. However, one important topic which is often overlooked during analysis and discussion is the type of backplane technology used in the display.
Display makers often throw around terms like A-Si, IGZO, or LTPS. But what do these acronyms actually mean and what’s the impact of backplane technology on user experience? What about future developments?
For clarification, backplane technology describes the materials and assembly designs used for the thin film transistors which drive the main display. In other words, it is the backplane that contains an array of transistors which are responsible for turning the individual pixels on and off, acting therefore as a determining factor when it comes to display resolution, refresh rate, and power consumption.Note the transistors at the top of each colored pixel.
Examples of backplane technology include amorphous silicon (aSi), low-temperature polycrystalline silicon (LTPS) and indium gallium zinc oxide (IGZO), whilst LCD and OLED are examples of light emitting material types. Some of the different backplane technologies can be used with different display types, so IGZO can be used with either LCD or OLED displays, albeit that some backplanes are more suitable than others.
a-Si
Amorphous silicon has been the go-to material for backplane technology for many years, and comes in a variety of different manufacturing methods, to improve its energy efficiency, refresh speeds, and the display’s viewing angle. Today, a-Si displays make up somewhere between 20 and 25 percent of the smartphone display market.A spec comparison of common TFT types.
For mobile phone displays with a pixel density lower than 300 pixels per inch, this technology remains the preferable backplane of choice, mainly due to its low costs and relatively simple manufacturing process. However, when it comes to higher resolution displays and new technologies such as AMOLED, a-Si is beginning to struggle.
AMOLED puts more electrical stress on the transistors compared with LCD, and therefore favours technologies that can offer more current to each pixel. Also, AMOLED pixel transistors take up more space compared with LCDs, blocking more light emissions for AMOLED displays, making a-Si rather unsuitable. As a result, new technologies and manufacturing processes have been developed to meet the increasing demands made of display panels over recent years.
LTPS
LTPS currently sits as the high-bar of backplane manufacturing, and can be spotted behind most of the high end LCD and AMOLED displays found in today’s smartphones. It is based on a similar technology to a-Si, but a higher process temperature is used to manufacture LTPS, resulting in a material with improved electrical properties.Higher currents are required for stable OLED panels, which a-Si falls short of.
LTPS is in fact the only technology that really works for AMOLED right now, due to the higher amount of current required by this type of display technology. LTPS also has higher electron mobility, which, as the name suggests, is an indication of how quickly/easily an electron can move through the transistor, with up to 100 times greater mobility than a-Si.
For starters, this allows for much faster switching display panels. The other big benefit of this high mobility is that the transistor size can be shrunk down, whilst still providing the necessary power for most displays. This reduced size can either be put towards energy efficiencies and reduced power consumption, or can be used to squeeze more transistors in side by side, allow for much greater resolution displays. Both of these aspects are becoming increasingly important as smartphones begin to move beyond 1080p, meaning that LTPS is likely to remain a key technology for the foreseeable future.LTPS is by far the most commonly used backplane technology, when you combine its use in LCD and AMOLED panels.
The drawback of LTPS TFT comes from its increasingly complicated manufacturing process and material costs, which makes the technology more expensive to produce, especially as resolutions continue to increase. As an example, a 1080p LCD based on this technology panel costs roughly 14 percent more than a-Si TFT LCD. However, LTPS’s enhanced qualities still mean that it remains the preferred technology for higher resolution displays.
IGZO
Currently, a-Si and LTPS LCD displays make up the largest combined percentage of the smartphone display market. However, IGZO is anticipated as the next technology of choice for mobile displays. Sharp originally began production of its IGZO-TFT LCD panels back in 2012, and has been employing its design in smartphones, tablets and TVs since then. The company has also recent shown off examples of non-rectangular shaped displays based on IGZO. Sharp isn’t the only player in this field — LG and Samsung are both interested in the technology as well.Smaller transistors allow for higher pixel densities
The area where IGZO, and other technologies, have often struggled is when it comes to implementations with OLED. ASi has proven rather unsuitable to drive OLED displays, with LTPS providing good performance, but at increasing expense as display size and pixel densities increase. The OLED industry is on the hunt for a technology which combines the low cost and scalability of a-Si with the high performance and stability of LTPS, which is where IGZO comes in.
Why should the industry make the switch over to IGZO? Well, the technology has quite a lot of potential, especially for mobile devices. IGZO’s build materials allow for a decent level of electron mobility, offering 20 to 50 times the electron mobility of amorphous silicon (a-Si), although this isn’t quite as high as LTPS, which leaves you with quite a few design possibilities. IGZO displays can therefore by shrunk down to smaller transistor sizes, resulting in lower power consumption, which provides the added benefit of making the IGZO layer less visible than other types. That means you can run the display at a lower brightness to achieve the same output, reducing power consumption in the process.
One of IGZO’s other benefits is that it is highly scalable, allowing for much higher resolution displays with greatly increased pixel densities. Sharp has already announced plans for panels with 600 pixels per inch. This can be accomplished more easily than with a-Si TFT types due to the smaller transistor size.
Higher electron mobility also lends itself to improved performance when it comes to refresh rate and switching pixels on and off. Sharp has developed a method of pausing pixels, allowing them to maintain their charge for longer periods of time, which again will improve battery life, as well as help create a constantly high quality image.
Smaller IGZO transistors are also touting superior noise isolation compared to a-Si, which should result in a smoother and more sensitive user experience when used with touchscreens. When it comes to IGZO OLED, the technology is well on the way, as Sharp has just unveiled its new 13.3-inch 8K OLED display at SID-2014.
Essentially, IGZO strives to reach the performance benefits of LTPS, whilst keeping fabrications costs as low as possible. LG and Sharp are both working on improving their manufacturing yields this year, with LG aiming for 70% with its new Gen 8 M2 fab. Combined with energy efficient display technologies like OLED, IGZO should be able to offer an excellent balance of cost, energy efficiency, and display quality for mobile devices.
What’s next?
Innovations in display backplanes aren’t stopping with IGZO, as companies are already investing in the next wave, aiming to further improve energy efficiency and display performance. Two examples worth keeping an eye are on are Amorphyx’ amorphous metal nonlinear resistor (AMNR) and CBRITE.Higher resolution smartphones, such as the LG G3, are putting increasing demands on the transistor technology behind the scenes.
Starting with AMNR, a spin-off project which came out of Oregon State University, this technology aims to replace the common thin-film transistors with a simplified two-terminal current tunnelling device, which essentially acts as a “dimmer switch”.
This developing technology can be manufacturing on a process that leverages a-Si TFT production equipment, which should keep costs down when it comes to switching production, whilst also offering a 40 percent lower cost of production compared with a-Si. AMNR is also touting better optical performance than a-Si and a complete lack of sensitivity to light, unlike IGZO. AMNR could end up offering a new cost effective option for mobile displays, while making improvements in power consumption too.
CBRITE, on the other hand, is working on its own metal oxide TFT, which has a material and process that delivers greater carrier mobility than IGZO. Electron mobility can happily reach 30cm²/V·sec, around the speed of IGZO, and has been demonstrated reaching 80cm²/V·sec, which is almost as high as LTPS. CBRITE also appears to lend itself nicely to the higher resolution and lower power consumption requirements of future mobile display technologies.LTPS vs CBRITE spec comparison for use with OLED displays
Furthermore, this technology is manufactured from a five-mask process, which reduces costs even compared to a-Si and will certainly make it much cheaper to manufacture than the 9 to 12 mask LTSP process. CBITE is expected to start shipping products sometime in 2015 or 2016, although whether this will end up in mobile devices so soon is currently unknown.
Smartphones are already benefiting from improvements in screen technology, and some would argue that things are already as good as they need to be, but the display industry still has plenty to show us over the next few years.
Color Filter Array (CFA)
Source: BASF
Source: BASF
Cathode ray tube television sets have long had their day. Flat screen TVs now provide energy-efficient, low-emission entertainment in three out of four German households, according to the Federal Statistical Office. And this figure is rising, Germans are estimated to have purchased eight million flat screen television sets in 2015, most of which are LCDs. LCD technology is also the basis for many other contemporary communication devices, including smartphones, laptops and tablets. After all, with experts forecasting six percent global annual sales growth for flat-panel displays until 2020.
LCD stands for liquid crystal display. Liquid crystals form the basis for billions of flat-panel displays. The American, George H. Heilmeier, unveiled the first monochrome LCD monitor to the expert community in 1968. Commercialization of the first color monitors took another 20 years. Flat screen TVs started sweeping the world in the 1990s, mainly because of the availability of high-performance color filter materials.
The images on a liquid crystal display with the standard resolution are made up of about two million picture elements, better known as pixels. The color filter pigments attached to the liquid crystal cells are what give each pixel its color. Screen contrast and color purity remain a challenge, however.
Pigment properties make all the difference
Red, green, and blue: Every pixel contains these three primary colors. The colors are composed of tiny crystals about a thousand times smaller in diameter than a human hair. The crystals act as a filter for the white backlight and only allow light waves from a selected range of the visible spectrum to pass through. These light waves show one of the three colors in its purest possible form. The filters block all the other wavelengths. “A good pigment has a significant impact on the brilliance of the colors the viewer sees,” said Dr. Hans Reichert, head of colorants research at BASF.
“Although perfect color selection is not feasible with absorbing materials, we come fairly close to perfection with our red filters.” Color purity also has an impact on the range of colors available. The greater the purity of the three primary colors, the more permutations that can be achieved by mixing them – and the more colorful the image.
The picture shows a chemical reaction in the lab yielding diketopyrrolopyrroles, the substances BASF’s red filter pigments are made of. Diketopyrrolopyrroles are aromatic organic ring compounds mainly consisting of carbon, nitrogen and oxygen.
The basic principle is simple. When the color red appears on the screen, the corresponding subpixel lets the red portion of light pass through and absorbs the rest. The other two subpixels – for blue and green – are deactivated when this happens. If, on the other hand, light penetrates through the red and green subpixel while the blue is deactivated, the colors combine to give a rich yellow. Fine-tuning the portions of the three primary colors in this manner produces millions of hues.
The liquid crystals fine-tune the blend of colors by twisting the plane of oscillation of the light waves. “This determines the brightness and color of the subpixels,” said Ger de Keyzer, in charge of applications engineering for color filter materials at BASF. “The liquid crystals change direction, and in that way alter their optical properties depending on the voltage applied.” They rotate the plane of oscillation of light waves to allow the light to pass through the second polarization filter. When an electrical field is applied, however, the crystals prevent some or all of the light from getting through.
To ensure that subpixels switch on and off the way they are supposed to, it is essential to prevent interferences from the color filter pigments. Any interferences resulting in scattering and depolarization of light will allow the light to pass uncontrolled through the filter. This contaminates the colors and compromises the contrast.
Smaller the better
“A good rule of thumb is: The smaller and more regular the crystals, the lower the scattering and the better the LCD image quality,” de Keyzer said. Researchers control the process mainly by managing the conditions in which pigment crystallization takes place. The underlying molecular structure is what determines which parts of the color spectrum are filtered out.
The organic red pigments that BASF manufactures consist mainly of carbon, nitrogen, and oxygen, and belong to the class of diketopyrrolopyrroles (DPPs). Blue and green pigments are phthalocyanine metal complex compounds. The raw product produced through chemical synthesis is mainly composed of irregular particles. They must then be brought into the ideal size and shape. This is done by a process called pigment finishing. Crystals that are too small are dissolved and precipitated onto the larger crystals. Crystals that are too large are broken into smaller pieces by a mechanical process until the balance is right. Dr. Roman Lenz, BASF lab team leader in charge of new color filter material synthesis, explained: “Our technology gives us color particles of 20 to 40 nanometers – small enough to reduce light scattering to an absolute minimum but large enough to provide a high degree of stability.” BASF has honed the technology almost to perfection with its products. The color particles in the latest generation of the Irgaphor® Red product suite are smaller than 0.00004 millimeters, and have double the contrast performance of their predecessors.
Tomorrow’s television screens will have to meet even higher expectations in terms of resolution and color purity. In anticipation of the new demands, Lenz and his colleagues are taking their lab experiments one step further. Their aim is to find new materials that will show colors in an even more natural light.
Source: STRUCTURE OF COLOR FILTERS/Toppan
Market Demands for Color Filters
Source: Toppan Japan
Source: STRUCTURE OF COLOR FILTERS/Toppan
Manufacturing Process of Color Filters
Source: Toppan Japan
Dyes and Pigments Used in Color Filters
Red Pigment/Dye
Green Pigment/Dye
Blue Pigment/Dye
High Transmittance, Low Scattering
Source: Past, present, and future of WCG technology in display
Dyes/Pigment Suppliers
DIC/Sun Chemicals. – Green and Blue
BASF – Red
Merck KGaA
Solvay
Clariant
Sumitomo Chemicals
Source: DIC Japan
Pigments for Color Filters Used in LCDs and OLED Displays(Functional Pigments)
Value Creation Global market-leading pigments that deliver outstanding brightness and picture quality
Color images on liquid crystal displays (LCDs) used in LCD televisions, computers and smartphones are produced using the three primary colors of light—red (R), green (G) and blue (B). These colors are created using pigments. LCDs produce images by transmitting light emitted from a backlight lamp through a color filter to which an RGB pattern has been applied. As a consequence, the pigments used in the color filter are crucial to picture quality. With Japan’s shift to digital terrestrial television driving up demand for flatpanel LCD televisions and the popularity of smartphones increasing, in 2007 DIC launched the G58 series of green pigments, which achieved a remarkable increase in brightness. The series includes FASTOGEN GREEN A350, a green pigment characterized by outstanding brightness and contrast that ensures excellent picture quality even with little light from the backlight. In fiscal year 2014, DIC developed the G59 series of green pigments for wide color gamut color filters, which deliver superior brightness and color reproduction, making them suitable for use in filters for next-generation high-definition displays, including those for ultra-high-definition (UHD) televisions. DIC currently enjoys an 85%- plus share of the global market for green pigments for color filters, making its products the de facto standard. DIC also manufactures blue pigments for color filters. In 2012, the Company developed the A series, which boasts a superb balance between brightness and contrast. The optical properties of pigments in this series have earned high marks from smartphone manufacturers and boosted DIC’s share of the global market for blue pigments to approximately 50%. DIC’s pigments for color filters, which satisfy the diverse performance requirements of displays used in LCD televisions, smartphones, tablets and notebook computers while at the same time adding value, have been adopted for use by many color filter manufacturers. In addition to improving picture quality, these pigments reduce energy consumption and, by extension, lower emissions of CO2. Having positioned pigments for color filters as a business that it expects to drive growth, DIC continues working to reinforce its development and product supply capabilities.
Applying technologies amassed through the production of printing inks to the development and expansion of functional pigments that have become the de facto standard worldwide
DIC first succeeded in developing offset printing inks in-house in 1915 and 10 years later began production of organic pigments for its own use. Over subsequent years, the Company amassed development and design capabilities, as well as production technologies, crucial to the manufacture of fine chemicals and in 1973 commercialized revolutionary high-performance, long-lasting nematic LCs, which were adopted by Sharp Corporation for use in the world’s first pocket calculator incorporating an LCD. DIC’s passion and development prowess are also evident in its pigments for color filters. Large-screen LCD televisions are expected to deliver superbly realistic and accurate color reproduction. The small LCDs used in smartphones and other devices must be clear, easy to read and bright enough to ensure legibility even with less light. This is because reduced light requirements results in longer battery life. Increasing brightness requires making color filters thinner and more transparent, but this alone will not deliver vivid colors and resolution. With the question of how best to realize both high brightness and vivid colors on ongoing challenge for display manufacturers, DIC has responded by developing innovative pigments for this application. Copper has traditionally been the central material used in green pigments. In developing its green pigments for color filters, DIC defied conventional wisdom by exploring the use of a different central material with the goal of further enhancing performance characteristics. Through a process of trial and error, the Company narrowed down the list of suitable materials from a wide range of candidates, eventually choosing zinc. DIC also significantly improved transparency by reducing the size of pigment particles, thereby achieving a dramatic increase in contrast, which ensures a bright, clear picture quality even with less light. The outcome of these efforts was the groundbreaking G58 series.
Picture quality is influenced significantly by the brightness and contrast of the pigment used in the color filter. (Left: High brightness and high contrast; Right: Low brightness and low contrast)
In the area of blue pigments for color filters, DIC also leveraged its superior molecular design capabilities to achieve outstanding tinting strength and precise particle size control. To develop the A series of blue pigments for color filters, the Company also employed specialty particle surface processing to ensure highly stable dispersion, realizing an excellent balance between brightness and contrast. Products in the A series currently dominate the market for blue pigments for color filters, delivering excellent optical properties that continue to earn solid marks from smartphone manufacturers. DIC’s success in developing a steady stream of pioneering functional pigments is supported by the seamless integration of basic technologies amassed in various fields as a manufacturer of color materials, the crossbusiness R&D configuration of its Central Research Laboratories and production technologies that facilitate the mass production of products with performance characteristics realized in the laboratory.
KEY PERSON of DIC
We are making full use of the DIC Group’s global network at all stages, from the promotion of product strategies through to the expansion of sales channels.
The value chain extending from functional pigments through to color filters for LCDs encompasses manufacturers of pigments, pigment dispersions, resist inks, color filters and LCDs. In developing pigments for color filters, we gather information on the latest trends from LCD manufacturers, which we apply to the formulation of nextgeneration product strategies. Production of pigment dispersions, color filters and LCDs is concentrated primarily in East Asia. Recent years have seen a particularly sharp increase in the People’s Republic of China (PRC), which is on the verge of overtaking the Republic of Korea (ROK) as No. 1 in terms of volume produced. We are making full use of the DIC Group’s global network by working closely with local Group companies to bolster the adoption of DIC pigments for color filters for use in LCDs.
Manager, Pigments Sales Department 2, Pigments Product Division Naoto Akiyama
Source: Emperor Chemicals China
Color filter (CF, COLOR FILTER) is one of the most important components of a color liquid crystal display, which directly determines the quality of the color image of the display. The rapid growth of LCD displays is supported by the strong demand for flat-panel color displays from notebooks (PCs, Personal Computers). The portable characteristics of the LCD, such as small outline size, thinness, lightness, high definition, and low power consumption, greatly meet the needs of notebook PCs. It is believed that in the multimedia age, TFT-LCD will have a huge advantage. Color filters are the key elements that make up a color image.
The color of the color filter may be dyed with a water-soluble dye, or a pigment dispersion method in which a pigment is colored. The pigment dispersion method includes the use of UV-curable phtoresists: colored pigments, UV-curable carrier resins, photo initiators, organic solvents, dispersants and other ingredients, among which organic pigments are colored The requirements for coloring properties of the agent, such as high vividness, specific primary color (RGB), three spectral hue, durability, chemical resistance and high transparency, etc., are mainly the selection of high-grade organic pigments through efficient dispersion Treatment process to obtain a pigment dispersion with a fine and stable particle size, and to prepare photoresist inks for color filters. Compared with the dyeing method, it has excellent moisture resistance, light fastness, and heat stability, but the pigment dispersion must be further improved Technology to prepare color filters with high transparency and pigment purity.
The color filter in the liquid crystal display adopts the principle of additive method, and uses blue, green and red organic pigments. Based on the spectral color and durability requirements of colorants, pigments for blue and green color photoresist inks are usually selected: phthalocyanine CI pigment blue 15: 1, pigment blue 15 :0, pigment blue 15: 3, Pigment Blue 15: 4, Pigment Blue 15: 6, and anthraquinone-based pigments such as CI Pigment Blue 60 and the like. Green tone C.I. Pigment Green 36.
In particular, the spectral absorption characteristics of CI Pigment Blue 15: 6 and CI Pigment Green 36 are well matched with the wavelengths and emission intensities of the blue, green, and red fluorescence emission spectra (fluorescence lamp for LCD backlight) in liquid crystal displays. In order to further improve the spectral characteristics, it is possible to adjust by adding a small amount of pigments of other colors, such as adding CI Pigment Violet 23 to obtain a stronger red light blue, and adding CI Pigment Yellow 150 to obtain a stronger yellow light green.
The selection of pigments should be based on obtaining a high-definition spectrum, eliminating unnecessary wavelength spectra, and retaining only the necessary color light. Selecting the organic pigment varieties required by the appendix, the color light purity and transmittance of the color filter can also be improved.
In order to adjust the spectral characteristics of the color filter, such as hue, tinting strength and contrast, for red, green and blue spectrum pigments, a second pigment component is often added to fight the color. For example, select some yellow with excellent durability, Purple organic pigment varieties, CI Pigment Yellow 138, CI Pigment Yellow 139, CI Pigment Yellow 150, CI Pigment Yellow 180, CI Pigment Purple 23 and other varieties.
Recommended organic pigments of three primary colors of red, blue and green are as follows:
Red organic pigments: The main varieties are high-grade organic pigments such as: C.I. Pigment Red 122, C.I. Pigment Red 177, C.I. Pigment Red 242, C.I. Pigment Red 254, and specific yellow organic pigment varieties are added if necessary.
Green organic pigments: C.I. Pigment Green 7, C.I. Pigment Green 36 is mainly selected, and specific yellow organic pigment varieties are matched, and specific yellow organic pigment varieties are added if necessary.
Blue organic pigments: C.I.Pigment Blue 5, C.I.Pigment Blue 15: 3, C.I.Pigment Blue 15: 6, C.I.Pigment Blue 60, etc., if necessary, specific yellow pigments and pigment violet 23.
Color Filter Less Technology
The liquid crystal display (LCD)is a thin, flat display device, which is made up of many number of color or monochrome pixels arrayed in front of a light source or reflector. It is prized for its superb image quality, such as low-voltage power source, low manufacturing cost, compared with other display device including CRT, plasma, projection, etc. Today the LCD device has been widely used in portable electronics such as cell phones, personal computers, medium and also in large size television display.
The LCD device consists of two major components, TFT-LCD panel and Back Light Unit (BLU). As LCD device can not light actively itself, thus a form of illumination, back light unit is needed for its display. While one of the key parts in LCD panel is color filter. The color filter is a film frame consists of RGB primary colors, and its function is to generate three basic colors from the back light source for LCD display. As a whole, back light and color filter are the two vital components of the perfect color display for LCD device.
Traditionally people use the cold cathode fluorescent lamp (CCFL)as the back light source for medium and large size LCD device. However CCFL has several disadvantages. For example, narrow color representation, low efficiency, complex structure, limited life, and the CCFL needs to be driven by a high-voltage inverter, consequently requires more space. Another disadvantage is the environmental problem for the mercury inside it. So people try to find an ideal back light module for LCD display.
Nowadays, the back light technology for LCD device towards the trend of using light emitting devices (LED). For its excellent advantages, the LCD device based on LED back light owns promoted display performance. As a new generation of solid-state light source, LED can produce very narrow spectrum, thus can generate a high color saturation, as a result it provides LCD device delivering a wider color gamut of above 100% of NTSC specification than the only 70% of CCFL back light. Moreover the LED only need DC power drive instead of a DC-AC inverter, so simplifies the back light structure. In a word, LED back light makes LCD obtain quite a higher display quality than the conventional CCFL back light. Despite of these advantages, there are also several challenges for LED back light technology currently, such as efficiency, stable ability, heat dissipation and cost etc. so people are trying to get some substantial breakthrough at the technical problems above to make LED back light as the key technology part for LCD device.
Color filter is another key component of the LCD device. As a sophisticated part, its fabrication takes an extremely complicated process, consequently the color filter occupies quite a large proportion of the production cost of the LCD devices. While a serious deficiency is its greatly influence on the light utilization rate. Generally speaking, only about 30% the amount of the light emitted from the back light can be delivered, while the rest of the light is wasted while passing the color filter.
For this, people prefer to designing a new form of LCD module which can get rid of the color filter, to promote the efficiency of light utilization. So an idea of Color Filter-Less (CFL) technology was put forward. The Field Sequential color LCD designed by Sumsang company is the first form of Color Filter-Less technology which is an idea of changing the space color mixing into the time color mixing.
Especially, we design a film frame which is patterned of red and green emitting phosphors, then make it be excited by blue light from a blue LED panel we fabricated. For its special emitting mechanism, this phosphor film can generate red and green emissions respectively. Meanwhile not all the blue light is absorbed by the phosphors, the remnant blue light can pass the film frame, therefore we can achieve a panel frame on which the RGB colors mixed together, thus to replace of the color filter in LCD device.
The following are the different types of RGB LEDs:
R/G/B/W – Has an additional white LED. This is often used where you need a pure white as well other combined colors.
RGB / 3 in 1 LED – Uses a red, a blue and a green LED chip are mounted within a common light engine and focused through a lens to produce a more uniform hue across the beam of light.
RGBW / 4 in 1 LED – similar to the RGB LED but with a warm white LED integrated in the light engine to offer more color tones.
White LED’s are actually blue leds with a yellow phosphor, and thus creating an white impression. This technique allows a colour gamut slightly wider than sRGB, but not very “colourfull”. RGB leds consist of 3 individual colour leds, red, green and blue. These allow an enourmous colour gamut that covers most standards like AdobeRGB and NTSC. Panels with RGB LED’s are much more expensive, as they need much more calibration logic. It is very hard to tame extreme gamut for say sRGB use, and the ballance of the colours is constantly monitored. RGB LED displays are doing twice the price of WLED’s with ease.
Composition of OLED Display
RGB OLED
White OLED
Source: Past, present, and future of WCG technology in display
This chapter discusses the color patterning technologies, which gives major contribution to cost and productivity. The technologies discussed include shadow mask patterning, white‐color filter method, laser‐induced thermal imaging method, radiation‐induced sublimation transfer method, and dual‐plate OLED display method. Low material utilization can bring high cost, so it is very critical to suppress material consumption during OLED display manufacturing. To address this, various high‐material‐utilization next‐generation OLED manufacturing processes, such as the vapor injection source technology (VIST) method, hot‐wall method, and organic vapor‐phase deposition (OVPD) have been proposed and are discussed in the chapter.
OLED Production: Composition and Color Patterning Techniques
Last updated on January 22, 2020
Organic Light-Emitting Diodes (OLEDs) are most famously known for their use in foldable smart phone displays. From the Samsung Galaxy Fold to the Huawei Mate X (2019), these devices offer huge screens that can fold down to the size of a more traditional smartphone screen. This revolutionary new technology is made possible by the properties and composition of OLED screens. In traditional Liquid Crystal Display (LCD) screens, a glass pane covers the actual liquid crystal display that emits the light. On the other hand, OLED screens have the light emitting technology already built into them. Thus, when you touch interact with an OLED device, you are touching the actual display too. OLED screens are often made of a type of plastic, which allows for flexibility and folding screens. These devices also require OLED color patterning techniques in order to integrate color into the display devices, which we will describe further in the upcoming sections.
Intro to OLED Composition
Now, we will brief on the composition and integration of OLED technology in this plastic screen. OLEDs are made of two or three organic layers sandwiched between two electrodes (cathode and anode) on top of a substrate layer. The organic layers and electrodes emit light in response to an electric current. One of the most difficult processes in manufacturing these OLEDs is attaching the organic layers to the substrate. For example, organic vapor phase deposition and inkjet printing are both efficient methods that can reduce the cost of producing OLED displays.
OLED components include organic layers that are made of organic molecules or polymers. This diagram is a two (organic) layer model. Courtesy of HowStuffWorks.
Another big part of OLED manufacturing is the color patterning step, which allows the OLED device to display color. There are various methods in use for OLED color patterning, including photolithography. Lithography is commonly used for semiconductors and TFTs, but presents challenges for OLEDs. This is due to the high temperature and humid conditions required for attaching OLED layers together. In this article we will explore three different color patterning technologies that have arisen for more efficient and accurate OLED optical manufacturing.
OLED Color Patterning and Masking Techniques
First, we have the “Shadow Mask Patterning Method” consists of placing red, green, and blue light emitting layers in a pattern in each pixel of the OLED device. Further, this has the advantage that each subpixel gets appointed a single, distinct color which produces great clarity of images. Unlike the other methods, there are no outer color filters required to produce the images. Thus, this method saves energy and is one of the most efficient. However, utilizing shadow masks can be an error filled process because the RBG subpixel pattern is outlined with a physical mask a.k.a stencil. We show an example of an accuracy error and its effects in the images below.
Error produced in processing an OLED with a red pixel mask. We can see that the spacing in the pattern is off around the red arrow. Courtesy of Tsujimura.
The resulting color variation in OLED screen due to shadow mask deformation. shown above. Courtesy of Tsujimura.
Second, we have the “Color Filter Method” a.k.a. “White+Color Filter Patterning” method. In this method, the OLED itself is also designed and manufactured with all three color elements in each pixel. However, different from the “Shadow Mask Patterning” method, these OLEDs only produce white light. Next, additional red, green, and blue color filters are utilized to match the desired color output. Accordingly, this process allows for a dynamic range of colors to be emitted with different levels of filtering. However, a big consequence of using color filters is that the purity of the image may be compromised due to interactions of the OLED light and physical color filters. Equally important is the high power consumption this method eats up. Because the color filters absorb most of the light intensity, the process requires a constant, powerful back light.
Schematic diagram of White+Color filter patterning method for OLEDs. Courtesy of Tsujimura.
Rising OLED Color Patterning Techniques: Electron Beams
In 2016, a new approach for OLED color patterning was developed at the Fraunhofer Institute for Organic Electronics. Researchers utilized electron beam technology to color pattern the organic layers in the OLED. Because this process acted on the micro-scale, it produced extremely accurate, high-resolution results, even with the help of color filters. Further, it also allowed for complex patterns and high-definition (HD) grayscale images. Since then, this technology has been developed and advanced to that of full-color working OLED displays, without the use of external filters.
Now, we will discuss an overview about how the electron beam process works. First, an OLED is produced containing all three RGB organic emitting layers. Akin to the previously mentioned processes, this OLED is designed to produce only white light. Next, a thermal electron beam is directed on the white emitting OLED. The electron beam excites certain molecules in the organic layer of the OLED, which causes the molecules or atoms to separate and become structured. Consequently, the thickness of different areas in the OLED organic layer changes and pixels with distinct colors (RGB) are formed. Moreover, the electron beam patterning process allows for microstructuring into color pixels without perturbing the other substrate and electrode layers.
Probe station with patterned OLEDs in the clean room. Courtesy of Fraunhofer FEP.
Conclusion
As more AMOLED, and flexible displays enter the market, OLED technology will continue to become more popular and widespread. One of the most important considerations for OLED availability in mass market, is in screen color production. Rising techniques such as the Electron Beam Patterning method can produce high quality, low cost, and energy efficiency. Another key consideration in OLED screen production is low material consumption. Further rising techniques in research that allow for low material costs include the vapor injection source technology (VIST) method, hot‐wall method, and organic vapor‐phase deposition (OVPD) [1].
This article is made possible by Gentec-EO, the market leader in the manufacture of light detection devices.
MicroLED is the next generation of display technology. Just like OLED, it produces its own light and therefore is capable of infinite contrast ratio. However, since it doesn’t use organic materials, it won’t deteriorate or burn-in over time.
What’s more, MicroLED displays will be brighter than OLED displays, and you will be able to customize their size, aspect ratio, and resolution (modular displays).
Mini-LED, on the other hand, improves on the existing LCDs by replacing their LED backlights with mini-LED backlights, which consist of more efficient and numerous light-emitting diodes that will increase contrast ratio, uniformity, response time, etc.
Although similar in name, microLED and mini-LED technologies are fundamentally diverse.
What is MicroLED?
MicroLED is the leading-edge display technology that is yet to be adjusted to the consumer market; in simpler terms, it’s the display technology of the not-so-distant future.
Similarly to OLED (Organic Light Emitting Diode) technology, MicroLED doesn’t rely on a backlight to produce light. Instead, it uses self-emissive microscopic LEDs, which allow for infinite contrast ratio, just like on OLED displays.
However, unlike OLED, MicroLED technology has no organic materials, so it won’t degrade over time, and you won’t have to worry about image burn-in.
Further, MicroLED displays are capable of higher luminance emission in comparison to OLEDs, which will allow for better details in highlights of the picture for a superior HDR (High Dynamic Range) viewing experience.
Lastly, they can have a unique modular characteristic that would allow you to customize the display’s screen size, resolution, and aspect ratio to your liking by arranging and connecting more panels together.Shop Related Products
What is Mini-LED?
Mini-LED technology improves on the existing LCDs.
It replaces their LED backlights with Mini-LED backlights, which consist of more LEDs that can offer a higher contrast ratio, better uniformity, faster response times, etc.
Mini-LED displays will be cheaper than OLEDs, but not better than them. So, Mini-LED is sort of a display technology in-between the standard LED-backlight LCDs and OLED displays.
The ASUS PG27UQX will feature 2,304 mini LEDs divided into 576 zones (4 LEDs per zone), whereas the original model has 384 zones for local dimming in comparison.
This will significantly alleviate one of the main issues of the PG27UQ, which is image bloom/halo.
When one zone is fully illuminated, but the zones surrounding it are dim, a certain amount of light will bleed from the lit zone to the dim zones, which generates the halo/bloom effect.
Since the PG27UQX has more zones, this issue will be decreased by ~33%. At the same time, the monitor will consume 7% less power and be (relatively) only slightly pricier than the PG27UQ model.
First there was LED (light emitting diode) display technology, commercialized in 1994. OLED (organic LED) products came on the market in 1997. Then microLEDs began to emerge in 2010. And now we’ve been hearing about a new display technology category: miniLEDs, poised to enter the market in 2019.1
As the name would imply, a miniLED is small—but not as small as a microLED (µLED). While there are no official definitions, microLEDs are typically less than 50 micrometers (µm) square, with most falling in the 3–15 µm size range. Generally, the term miniLED (sometimes also called “sub-millimeter light emitting diodes”) refers to LEDs that are roughly 100 µm square (0.1 mm square), although “mini” can also simply describe any LED between micro and traditional size.
LED landscape as of 2018. Image Source: “MiniLED for Display Applications: LCD and Digital Signage” report by Yole Développement, October 2018.
Though they share many similarities, miniLEDs and microLEDs are also different in some key ways. MicroLEDs are not just shrunken versions of their miniLED sisters. The two LED types have different performance and structures. LEDinside characterized the difference as follows: “Micro LED is a new-generation display technology, a miniaturized LED with matrix. In simple terms, the LED backlight is thinner, miniaturized, and arrayed, with the LED unit smaller than 100 micrometers. Each pixel is individually addressed and driven to emit light (self-emitting), just like OLED…Mini LED is a transitional technology between traditional LED and Micro LED, and is an improved version of traditional LED backlight.”2
Additionally, a driving factor in the recent emergence of miniLEDs is that they are less expensive to produce, largely because current fabrication facilities can more quickly be switched over to miniLED production. MiniLEDs are essentially a variation of already mature LED technology.
MicroLED Fabrication Challenges
MicroLEDs are typically made from Gallium-nitride-based LED materials, which create brighter displays (many times brighter than OLED) with much greater efficiency than traditional LEDs. This makes them attractive for applications that need both brightness and efficiency such as smart watches, and particularly for head-up displays (HUDs) and augmented reality systems that are likely to be viewed against ambient light backgrounds
OLED screen manufacturing has been somewhat costly to date, limiting its adoption primarily to smaller screen sizes like smart phones. Likewise, producing an entire television screen out of microLED chips has so far proven to be challenging. MicroLEDs require new assembly technologies, die structure, and manufacturing infrastructure. For commercialization, fabricators must find methods that yield high quality with microscopic accuracy while also achieving mass-production speeds. For starters, a miniLED backlight screen may be made up of thousands of individual miniLED units; a microLED screen is composed of millions of tiny LEDs.
To fabricate a display, each individual microLED must be transferred to a backplane that holds the array of units in place. The transfer equipment used to place microLED units is required to have a high degree of precision, with placement accurate to within +/- 1.5 µm. Existing pick & place LED assembly equipment can only achieve +/- 34 µm accuracy (multi-chip per transfer). Flip chip bonders typically feature accuracy of +/-1.5 µm—but only for a single unit at a time. Both of these traditional LED transfer methods are not accurate enough for mass production of microLEDs.
New transfer solutions are under development, including fluid assembly, laser transfer, and roller transfer. Researchers are also working to resolve the challenges associated with integrating compound semiconductor microLEDs with silicon-based integrated circuit devices that have very different material properties and fabrication processes. Traditional chip bonding and wafer bonding processes don’t provide efficient mass transfer for microLED, so various thin-film-transfer technologies are being explored.
Despite Samsung’s introduction of a prototype 75-inch microLED television at the recent CES show (below), microLED products are not expected to reach the general market until 2021.3
Han Jong-hee, president of Samsung Electronics’s video display business, introduces a new 75-inch microLED TV in Las Vegas on January 6, 2019. Photo Source: Business Korea
MiniLED Advantages
By contrast, miniLED chips do not present similar production complications. Because they are just smaller versions of traditional LEDs, they can be manufactured in existing fabrication facilities with minimal reconfiguration. This ease means miniLED production is already underway and devices will reach the market this year for applications in gaming displays and signage, followed by backlight products such as smartphones, TVs, virtual reality devices, and automotive displays.
For example, miniLEDs can be used to upgrade existing LCD displays with “ultra-thin, multi-zone local dimming backlight units (BLU) that enable form factors and contrast performance”4 that rival the quality of OLED displays. MiniLEDs also have an advantage as a cost-effective solution for narrow-pixel-pitch LED direct-view displays such as indoor and outdoor digital signage applications.
MiniLED backlight television from Chinese manufacturer TLC displayed at CES 2019. Photo Source: FlatpanelsHD.
MicroLEDs do offer high luminous efficiency, brightness, contrast, reliability and a short response time, but they are likely to be priced at more than three times traditional LED screens during initial the initial stages of mass production. MiniLEDs, while they perform more like traditional LEDs, do have advantages when it comes to HDR and notched or curved display designs, and could launch at just 20% above standard LCD panel prices.5 According to PCWorld, “at this stage, the biggest difference between microLED and miniLED for consumers is that microLED is likely to make it to market as a fully-fledged next-generation display technology of its own while miniLED is likely to mostly be used by manufacturers to enhance existing display technologies.”6
Together, microLEDs and miniLEDs are expected to have roughly equal shares of a $1.3 billion market by 2022.7
Quality Assurance for All LED Types
Whether LED or OLED, micro- or mini-, LED display products of all types are jostling for room in a highly competitive marketplace, where customers expect a perfect viewing experience right out of the box. Defects, variations in color or brightness, and other irregularities can quickly deflate buyer satisfaction, hurt brand reputation, and erode market share.
To ensure the absolute quality of OLED- and LED-based devices, Radiant’s ProMetric® Imaging Photometers and Colorimeters measure display performance and uniformity down to the pixel and subpixel level, matching the acuity and discernment of human visual perception.
CITATIONS:
YiningChen, “Mini LED Applications to be Launched in 2019 and Micro LED Displays in 2021.” LEDinside, October 19, 2018. LINK
Evangeline H, “Difference between Micro LED and Mini LED.” LEDinside,May 8, 2018. LINK
YiningChen, “Mini LED Applications to be Launched in 2019 and Micro LED Displays in 2021.” LEDinside, October 19, 2018. LINK
“MiniLED for Display Applications: LCD and Digital Signage” report by Yole Développement, October 2018, as reported in “Mini-LED adoption driven by high-end LCD displays and narrow-pixel-pitch LED direct-view digital signage”. Semiconductor Today, November 28, 2018. LINK
Evangeline H, “Difference between Micro LED and Mini LED.” LEDinside,May 8, 2018. LINK
Halliday, F. “MicroLED vs Mini-LED: What’s the difference?” PCWorld, September 11, 2018. LINK
YiningChen, “Micro LED & Mini LED Market Expects Explosive Business Opportunities, with an Estimated market Value of $1.38 Billion by 2022”. LEDinside (a division of market research company TrendForce), June 20, 2018. LINK
LED TV, QLED TV with QDEF-CF, and QLED TV with QD-CF
Source: Environmentally friendly quantum-dot color filters for ultra-high-definition liquid crystal displays
Source: Samsung Displays – Public Information Display
QLED – Quantum Dot LED
QLED stands for Quantum Dot Light-Emitting Diode, also referred to as quantum dot-enhanced LCD screen. While similar in working principle to conventional LCDs, QLEDs are using the properties of quantum dot particles to advance color purity and improve display efficiency. Quantum dots are integrated with the backlight system of the LCD screen, most commonly with the help of Quantum Dot Enhancement Film (QDEF) that takes place of the diffuser film. Blue LEDs illuminate the film, and quantum dots output the appropriate color, based on their size.
OLED – Organic LED
OLED stands for Organic Light-Emitting Diode, which is self-emitting. Not all OLEDs are using the same tech though. The OLED technology used in phone screens is RGB-OLED, which is completely different from the White OLED (also referred to as W-OLED) used in TVs and large format displays.
RGB-OLED vs. White OLED
RGB-OLEDs use individual sub-pixels emitting red, green, and blue light. RGB-OLEDs yield excellent color reproduction but are unfit for performance requirements of large format displays. With the evolution of materials and a difference in use cases comparing to TVs, RGB-OLED is a preferred technology for the smartphone use.
White OLEDs, in turn, emit white light, which then is passed through a color filter to generate red, green, and blue—similar to how LCDs function. Modern W-OLED color filters use RGBW (red, green, blue, white) structure, adding an additional white sub-pixel to the standard RGB to improve on the power efficiency, enhance brightness, and to mitigate issues with the OLED burn-in. Although having more complex circuit requirements than LCDs (emission is current-driven rather than voltage-driven), W-OLEDs can be utilized for large-scale displays.
Quantum Dot OLED TVs are expected to finally go real in 2021. As the name suggests, these TV displays will use Quantum Dot technology to enhance and improve the existing OLED panels.
How exactly are QD-OLED displays different from current OLED display panels manufactured by LG Displays and from Samsungs existing QLED TVs? The next year will also see a surge in mini LED TVs which will be priced a little below OLED TVs. So let’s compare these different TV technologies to better understand which one is better and why.
OLED on TVs and OLED on Phones are not the same
To understand the difference between these display technologies and why they exist, it must first be cleared that the OLED displays on TVs are not the same as OLED displays on phones.
On your phones, the OLED panels have red, green, and blue subpixels that are self-emissive or emit their own red, green, and blue colors – and can be individually powered on or powered off.
Making similar OLED panels for large TVs with individual Red, Green and Blue subpixels, however, poses several manufacturing and longevity challenges. In fact, only one such TV was ever launched – the Samsung KE55S9C 55-inch UHD OLED- which was introduced in 2013.
Samsung KE55S9C 55-inch UHD OLED TV with true RGB colors
The technology wasn’t scalable for larger resolution or bigger displays and thus Samsung shifted to Quantum Dots based QLED technology for its premium TVs.
Meanwhile, LG Displays developed OLED for TVs where all subpixels are white and not RGB.
The white OLED light is achieved by using Blue and Yellow substrate. Different colors for four sub-pixels (R, G, B, W) are achieved by using a RGBW color filter layer over the essentially white OLED subpixels. This works because a single color OLED panel is easier to manufacture and decays uniformly – which is to say that your TV will age to be less bright but the backplane light shall still remain uniformly blue or uniformly yellow.
The color filter film used in front of OLED subpixels, however, is not an ideal solution. The filters work by blocking particular colors of light thus reducing brightness, and as the Blue OLED material decays over time, Red, Green, and Blue colors are affected differentially (the decay is not the same for all three colors resulting in color shifts, burn-in and other issues).
QD OLED or Quantum Dot OLED TVs aim to fix these issues by using a quantum dots layer for color conversion instead of a color filter.
What are Quantum Dots and why they are better than color filters?
Quantum dots are small nanocrystals. When a high-energy light photon strikes quantum dots, they absorb it and emit a new photon. The color of this emitted photon depends on the size of the quantum dot – so manufacturers have to use the same material (just different sizes) for all colors, which makes manufacturing simpler and helps with uniform aging.
Source: Nanosys
In TVs, Quantum Dots are excited by higher energy or lower wavelength light than the emission color of the dot. To excite green and red color quantum dots, TV manufacturers thus use blue light and for blue subpixels, they let the blue light pass through as-is.
The same result can perhaps be obtained by using blue, red and green quantum dots and exciting them using ultraviolet light. However, Blue quantum dots are not as easy to develop as green and red (Samsung does have blue Quantum dot technology, but it is not yet being used commercially).
Quantum dots act as an excellent color converter and have almost 100 percent quantum efficiency. Thus unlike color filters, the Quantum dots layer doesn’t block lights of particular wavelengths or colors and let the entire luminance pass through.
QD-OLED vs OLED: Why QD-OLED displays are better
QD-OLED TV Layers (source: Nanosys)OLED TV layers
As mentioned above, color conversion in QD-OLED displays is done by quantum dots that are placed or patterned at a sub-pixel level over Blue OLEDs.
So, we have a blue emissive layer in the backplane where all pixels are blue. And then green and red quantum dot materials are printed on pixels that are needed to be green or red.
White OLED vs QD -OLED (Source: Nanosys)
Colors are converted on red sub-pixels by red quantum dots and green sub-pixel by green quantum dots. Using this technology, the end result is similar to what you’d get with individual Red, Green, and Blue sub-pixels as with AMOLED displays on phones.
QD-OLED vs OLED color gamut
Quantum Dots as color converters are highly efficient and way better than color filters that can block up to 60% light.
Another benefit of this implementation over color filter is that as the Blue OLED lights get dimmer with time, the red and green light getting out of the quantum dots will dim proportionally.
So, over the lifetime of your TV, its display may get less bright but colors shall remain mostly unaffected. The use of Quantum dot also helps with wider color gamuts with fewer image artifacts, better brightness, and better HDR.
OLED TVs today use LG Display panels that have a white pixel along with red, green, and blue sub-pixels (and are also referred to as White OLED). This is used for enhancing brightness but reduces color vibrance. Upcoming QD-OLED panels will, in a way, re-instate RGB OLED with deeper, brighter, and more vibrant colors.
OLED technology is known to have problems with aging, but the current crop of OLED TVs handle this remarkably well. There are negligent chances that users will face issues like OLED burn-ins over a life span of 5 to 8 years.
Disadvantages of QD-OLED Displays
We discussed a few theoretical advantages of QD OLED above and let’s now talk about some disadvantages of the technology.
Samsung is currently developing QD OLED panels that we will see in the first wave of QD OLED televisions and they won’t be perfect.
One problem is that Quantum dots on the QD OLED TVs get excited by UV light falling on the TV from the outside. Secondly, Quantum Dot color conversion materials don’t always capture the entire blue light that is used to excite them and some of it may bleed into Red and Green subpixels.
To counter these problems, Samsung Displays is likely to use some sort of color filter which is likely to be eliminated as we progress to second or third-generation QD-OLED panels. It remains to be seen how much brightness penalty is incurred meanwhile.
QLED vs QD-OLED: What’s different?
QLED layers
Now that we have discussed how Quantum dots are enhancing existing OLED TVs, you might be wondering how the Quantum Dot technology is implemented on existing Samsung QLED TVs.
Unlike QD-OLED TVs, QLED TVs use Quantum dots as a backplane technology behind the LCD.
A QLED TV works just like LCD TVs, but a Quantum Dot Enhancement Film( QDEF) is used in front of the Blue LED backlight to convert portions of the blue light to Red and Green in order to get pure White light. This helps enhance brightness and achieve a wider color gamut for better HDR performance.
QLED TVs are better at avoiding the backlight bleed into the display colors as compared to conventional LED or mini LED TVs. Samsung’s high-end QLED models can also get brighter than TV OLED displays. Color conversion is still done using a color filter in front of the LCD module.
And what about Mini LEDs?
LED TVs don’t have self-emissive pixels and it’s not possible to turn off individual pixels. The LCD substrate merely blocks the white light from the backlight to portray blacks, resulting in slightly greyish blacks more noticeable in dark ambiance. The contrast and black level can however be improved by turning off a portion or zone of the backlight.
That’s where mini LED TVs come in. These TVs have an array of mini LEDs behind the screen which can be individually turned off for a section of the screen. These mini LEDs don’t map pixels one to one, but having more zones helps with better local dimming control and thus enhances quality over conventional edge-lit LED displays.
Manufacturers are working on adding quantum dot enhancements to microLED backlighting as well (similar to QD enhancements in QLED TVs).
When will we see QD OLED TVs?
Samsung Displays is manufacturing QD-OLED displays but Samsung Electronics isn’t keen on adopting the technology. That’s because Samsung has been marketing QLED as superior to OLED panels for years and transitioning back to OLED or OLED-based TVs will make them lose face.
QD OLED panels are however being provided to a number of other manufacturers including Sony and we will most probably see QD-OLED TVs in 2021.
QD-BOLED
Source: Inkjet printed uniform quantum dots as color conversion layers for full-color OLED displays
Quantum dots (QDs) have shown great potential for next generation displays owing to their fascinating optoelectronic characteristics. In this work, we present a novel full-color display based on blue organic light emitting diodes (BOLEDs) and patterned red and green QD color conversion layers (CCLs). To enable efficient blue-to-green or blue-to-red photoconversion, micrometer-thick QD films with a uniform surface morphology are obtained by utilizing UV-induced polymerization. The uniform QD layers are directly inkjet printed on red and green color filters to further eliminate the residual blue emissions. Based on this QD-BOLED architecture, a 6.6-inch full-color display with 95% Broadcasting Service Television 2020 (BT.2020) color gamut and wide viewing-angles is successfully demonstrated. The inkjet printing method introduced in this work provides a cost-effective way to extend the applications of QDs for full-color displays.
Liquid Crystal Display: Environment & Technology Ankita Tyagi1, Dr. S. Chatterjee 2
1Centre for Development of Advanced Computing, New Delhi, India 2 Department of Electronics and Information Technology Ministry of Communication and Information Technology New Delhi, India
Jana Olson, Alejandro Manjavacas, Lifei Liu, Wei-Shun Chang, Benjamin Foerster, Nicholas S. King, Mark W. Knight, Peter Nordlander, Naomi J. Halas, and Stephan Link
Past, present, and future of WCG technology in display
Musun Kwak | Younghoon Kim | Sanghun Han | Ahnki Kim | Sooin Kim | Seungbeom Lee | Mike Jun | Inbyeong Kang
Color Team, Panel Performance Division, LG Display, LG Science Park, Seoul, Korea
Musun Kwak, Color Team, Panel Performance Division, LG Display, LG Science Park, Magokjungang, Gangseogu, 10‐ro, Seoul, Korea. Email: musunkwak@lgdisplay.com
Synthesis of yellow pyridonylazo colorants and their application in dye–pigment hybrid colour filters for liquid crystal display
Jong Min Park, Chang Young Jung, Wang Yao, Cheol Jun Song and Jae Yun Jaung*
Department of Organic and Nano Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 133791, South Korea Email: jjy1004@hanyang.ac.kr
Received: 9 June 2015; Accepted: 29 September 2015
Quantum Dot Conversion Layers Through Inkjet Printing
Ernest Lee, Ravi Tangirala, Austin Smith, Amanda Carpenter, Charlie Hotz, Heejae Kim, Jeff Yurek, Takayuki Miki*, Sunao Yoshihara*, Takeo Kizaki*, Aya Ishizuka*, Ikuro Kiyoto*
Preparation of Colour Filter Photo Resists for Improving Colour Purity in Liquid Crystal Displays by Synthesis of Polymeric Binder and Treatment of Pigments
Chun Yoon* and Jae-hong Choi‘
Department ofChemistry, Sejong University, Seoul 143-747, Korea. *E-mail: chuny@sejong.ac.kr ‘Department of Textile System Engineering, Kyungpook National University, Daegu 702-701, Korea Received May 04, 2009, Accepted July 03, 2009
1CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA 2NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, USA
Full-color micro-LED display with high color stability using semipolar (20-21) InGaN LEDs and quantum-dot photoresist
SUNG-WEN HUANG CHEN,1 YU-MING HUANG,1,2 KONTHOUJAM JAMES SINGH,1 YU-CHIEN HSU,1 FANG-JYUN LIOU,1 JIE SONG,3 JOOWON CHOI,3 PO-TSUNG LEE,1 CHIEN-CHUNG LIN,2 ZHONG CHEN,4 JUNG HAN,5 TINGZHU WU,4,6 AND HAO-CHUNG KUO1,7
Sourcez: AN UPDATE ON COLOR IN GEMS. PART 1: INTRODUCTION AND COLORS CAUSED BY DISPERSED METAL IONS
Three most common causes of color in gem materials:
Dispersed metal ions
Charge transfers and other processes that involve multiple ions, and colorcenters.
Coloration that are less often seen in gems, such as those that result from physical phenomena (asin opal) or from semiconductor-like properties (as in natural blue diamond).
The most common cause of color in gemstones is the presence of a small amount of a transition metal ion. These transition metal ions have an incomplete set of 3d electrons. Changes in the energy of these electrons correspond to the energy of visible light. When white light passes through a colored gemstone or is reflected by it, some of the energy of the visible light is absorbed, causing 3d electrons in the transition metal ion to undergo an energy change. The light that is transmitted or reflected appears colored, because those colors corresponding to 3d– electron energy transitions have been absorbed. The table lists several common gemstones, their chemical compositions, colors, and the origins of these colors.
A ruby is a crystal of alumina, aluminum oxide, containing a trace of chromium(III) ions replacing some of the aluminum ions. In ruby, each Al3+ ion and Cr3+ ion is surrounded by six oxide ions in an octahedral arrangement.
Gem
Formula
Color
Origin of color
Ruby
Al2O3
Red
Cr3+ replacing Al3+ in octahedral sites
Emerald
Be3Al2(SiO3)6
Green
Cr3+ replacing Al3+ in octahedral site
Alexandrite
Al2BeO4
Red/Green
Cr3+ replacing Al3+ in octahedral site
Garnet
Mg3Al2(SiO4)3
Red
Fe2+ replacing Mg2+ in 8- coordinate site
Peridot
Mg2SiO4
Yellow-green
Fe2+ replacing Mg2+ in 6- coordinate site
T ourmaline
Na3Li3Al6(BO3)3(SiO3)6F4
Pink
Mn2+ replacing Li+ and Al3+ in octahedral site
Turquoise
Al6(PO4)4(OH)84H2O
Blue-green
Cu2+ coordinated to 4 OH and 2 H2O
Sapphire
Al2O3
Blue
Intervalence transition between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
This arrangement splits the five 3d orbitals of Cr3+ into two sets, the dxy, dxz, dyz orbitals and the dx2-y2 and dz2 orbitals. These two sets have different energies. The energy difference between these sets corresponds to the energy of visible light. When white light strikes a ruby, the gem absorbs the light of energy corresponding to the transition of an electron from the lower-energy set of 3d orbitals to the higher-energy set. The ruby reflects or transmits the remainder of the light. Because this light is deficient in some energies (those that were absorbed), the light appears colored.
The origin of the color of emeralds is similar to that of the color of rubies. However, the bulk of an emerald crystal is composed of beryl, beryllium aluminum silicate, instead of the alumina which forms rubies. The color is produced by chromium(III) ions, which replace some of the aluminum ions in the crystal. In emeralds, the Cr3+ is surrounded by six silicate ions, rather than the six oxide ions in ruby. These silicate ions also split the 3d orbitals of Cr3+ into two sets. However, the magnitude of the energy difference between the sets is different from that produced by the oxide ions in ruby. Therefore, the color of emeralds is different from that of ruby.
Chromium(III) also produces color in alexandrite. The color of this gem is very unusual, because in bright sunlight it appears green, but in incandescent light it appears red. This unusual behavior is a result of the way human vision works. Our eyes are most sensitive to green light. Alexandrite reflects both green and red light. In bright sunlight, the proportion of green light is greater than it is in the light from an incandescent lamp. The light reflected by alexandrite in bright sunlight is rich in green light, to which our eyes are most sensitive, and we perceive the gem as green. The light reflected by alexandrite in incandescent light is much richer in red, and we see the stone as red under these conditions.
Energy transition of the 3d orbitals of other transition metal ions are responsible for the colors of other gemstones. Iron(II) produces the red of garnets and the yellow-green of peridots. Manganese(II) is responsible for the pink coloration of tourmaline, and copper(II) colors turquoise.
In some gemstones, the color is caused not by energy changes in a single transition metal ion, but by the exchange of electrons between two adjacent transition metal ions of differing oxidation states. The energy needed to transfer an electron from one ion to another corresponds to the energy of visible light. An example is sapphire. The bulk of sapphire is alumina, as in rubies, but some adjacent pairs of Al3+ ions are replaced by an Fe2+ ion and a Ti4+. When light of the appropriate energy strikes the crystal, energy is absorbed, and an electron moves from the Fe2+ to the Ti4+. Such a movement is called an intervalence transition. An intervalence transition is also responsible for the blue color of aquamarine. In aquamarine, adjacent Al3+ ions in beryl are replaced by an Fe2+ ion and an Fe3+ ion.
Not all gem colors are produced by transition metal ions. In some gemstones, the colors are produced by the presence of foreign atoms with a different number of valence electrons than the ones they replace. These foreign atoms are called color centers. Because the replacement atoms have the wrong number of valence electrons, they can supply or receive an electron from another atom by an intervalence transition. These color centers are often produced by nuclear transformation. An example of such a transformation is the change of a radioactive carbon- 14 atom in diamond into a nitrogen atom through beta particle emission. This leaves an atom of nitrogen in place of the original carbon atom. The nitrogen atom has one more valence electron than the carbon atom. These nitrogen atoms are the cause of the coloration of blue and yellow diamonds. Color centers can be caused artificially as well, by irradiating the gem in a nuclear reactor. Many bright blue and bright yellow diamonds are produced artificially in this manner.
REFERENCES
Chemistry in Britain, 1983, page 1004. Gems and Gemology, Volume 17, 1981, page 37. Scientific American, October 1980, page 124.
Precious Stones
The Diamond
The Pearl
The Ruby
The Sapphire
The Emerald
The Oriental Cateye
The Alexandrite
RGB Colors of Gemstones
Blue Sapphire
Emerald
Ruby
Pearl
Tahitian Cultured Pearls
Diamond
Chrysoberyl (Oriental Cat’s Eye)
Alexandrite
Change in Color due to change in Illuminant
Semi Precious stones
The Amethyst
The Topaz
The Tourmaline
The Aquamarine
The Chrysoprase
The Peridot
The Opal
The Zircon
The Jade
The Garnet
The Lapis lazuli
The Moonstone
The Spinel
The Turquoise
The Agate
The Coral
The Citrine
The Onyx
The Chrysolite
The Amber
The Chrysoberyl
The Chalcedony
The Morganite
The Quartz
The Tanzanite
Amethyst
Topaz
London Blue Topaz
Blue Topaz
Tourmaline
The Aquamarine
Chrysoprase
The Peridot
The Opal
The Zircon
The Jade
Garnet
Lapis lazuli
The MoonStone
White Moonstone
Grey Moonstone
The Spinel
Turquoise
Agate
Red Agate
Citrine
Onyx Black
Chalcedony
Rose Quartz
Color Chemistry of Gemstones
Healing Power of Gemstones and Crystals
Precious Stones and Semi Precious Stones arranged by Color
Precious and Semi Precious Stones and their characteristics
Birthstones by Month
Source: AN UPDATE ON COLOR IN GEMS. PART 3: COLORS CAUSED BY BAND GAPS AND PHYSICAL PHENOMENA
Source: Structural color and its interaction with other color-producing elements: perspectives from spiders
Christmas Tree
Multilayer – 1 D Periodicity
Photonic Crystals – 2 D and 3 D
Diffraction Grating
Quasi Ordered Photonic Crystal
Disorder Structure
Source: BIO-INSPIRED VARIABLE STRUCTURAL COLOR MATERIALS
1 D Gratings
1 D Periodicity Multilayers
1 D Discrete Periodicity
2 D Gratings
2 D Periodicity
Closed Packed Spheres of Solid Materials
Inverse Opal Analogoues
Source: STRUCTURAL COLORATION IN NATURE
Thin Film Interference
Multi Film Interference
Diffraction Gratings
Coherent Scattering
Incoherent Scattering
1 D Photonic Crystals
2 D Photonic Crystals
3 D Photonic Crystals
Source: Structural Color and Odors: Towards a Photonic Crystal Nose Platform
Source: PHYSICS OF STRUCTURAL COLORS
Source: PHOTOPHYSICS OF STRUCTURAL COLOR IN THE MORPHO BUTTERFLIES
Source: Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)
Cuticular Multilayer Reflector
Epicuticular Reflector
Exocuticular Reflector
Endocuticular Reflector
Source: Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)
Multilayer Reflectors
Diffraction Gratings
3 D Photonic Crystals
Multilayer reflectors in beetles have also been described as ‘thin-layer stacks’, ‘one-dimensional photonic crystals’ and ‘thin-film reflectors’ (e.g. Parker 1998, 2002; Vigneron et al. 2006). The vocabulary used to describe these structures is somewhat dispersive, as the variously intersecting disciplines of entomology, physics and applied optics (e.g. laser technology, fibre-optic data transmission, telescopes and microscopy) have all developed slightly different suites of terminology. Other synonyms for ‘multilayer reflector’ include multilayer stack, quarter wave stack, interference reflector and dielectric mirror.
We propose that the term multilayer reflector be applied to such structures in Coleoptera; this describes the multilayered nature of cuticular chitin lamellae (which are not true films) and the reflective mechanism by which colour is produced.
The terms ‘metallic colours’ or ‘metallic iridescence’ can be used to distinguish multilayer effects from those produced by other optical structures. Multilayer reflectance can typically be diagnosed as such by its limited palette (usually one or two apparent hues per reflector), blue shift with decreased observation angle and fixed position on the cuticle surface.
Source: Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)
Three-dimensional crystalline structures producing scintillating, gem-like reflectance were described by Parker et al. (2003) in the entimine weevil Metapocyrtus sp. (initially misidentified as Pachyrrhynchus argus); by Welch et al. (2007) in Pachyrrhynchus congestus, and recently in another entimine weevil, Lamprocyphus augustus, by Galusha et al. (2008). The photonic crystals found in the scales of pachyrrhynchine weevils (Pachyrrhynchus and Metapocyrtus) have a close-packed hexagonal arrangement analogous to (mineral) opal, while the photonic crystal of Lamprocyphus has a diamond-based lattice (i.e. a face-centred cubic system rather than a hexagonal one).
Although the term ‘photonic crystal’ applies to any ordered subwavelength structure that affects the propagation of specific wavelengths of light (Parker & Townley 2007), it is the three-dimensionally ordered structures to which the term is most commonly applied. We recommend use of the term ‘three-dimensional photonic crystal’, which distinguishes these structures from the one-dimensional periodicity of multilayer reflectors or Bragg gratings. The terms ‘opal’ and ‘diamond based’ have been used to describe iridescence in weevil scales, but refer to phenomena that are relatively similar from an organismal perspective; it is important to note that these terms refer to crystalline lattice morphology and not the appearance of the scales themselves. Maldovan & Thomas (2004) provided an excellent overview of diamond-based lattice morphology (as observed in Lamprocyphus) in photonic crystals; Yablonovitch (1993) provided a thorough introduction to the photonic band-gap mechanism by which colours are produced in three-dimensional photonic crystals.
Source: Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)
A diffraction grating is any nanoscale array of parallel ridges or slits that disperses white light into its constituent wavelengths (figure 8a shows a grating in cross section). Because white light consists of many different wavelengths, it diffracts into full spectra, creating the rainbow-like reflectance shown in figures 1a,b, 8c and 9b,d. While man-made diffraction gratings can disperse light via reflection or transmission, all beetle gratings are strictly reflection mechanisms.
Source: Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)
Nature’s Fantastical Palette: Color from Structure
Philip Ball 18 Hillcourt Road East Dulwich London SE22 0PE, UK p.ball@btinternet.com
The changing hues of a peacock’s splendid tail feathers have always captivated the curious mind (Figure 1). The seventeenth-century English scientist Robert Hooke called them ‘fantastical’ because the colors could be made to disappear by wetting the feathers (Hooke, 1665). Using the newly invented microscope, Hooke looked at peacock feathers and saw that they were covered with tiny ridges, which he figured might be the origin of the colors.
Figure 1. The shifting colors of the peacock’s tail have had metaphorical interpretations for centuries.
Hooke was on the right track. The bright, often iridescent colors of bird plumage, insect cuticle and butterfly wings are ‘structural’; produced not by light absorption by pigments, but light scattering from a regular array of objects just a few hundreds of nanometers (millionths of a millimeter) in size (Vukusic & Sambles, 2003; Vukusic, 2004; Wolpert, 2009). This scattering favors particular wavelengths depending on the size and spacing of the scatterers, and so it picks out specific colors from the full spectrum of sunlight. Because the precise hue may depend also on the viewing angle, structural colors are often iridescent, changing from blue to green or orange to yellow. And because they involve reflection rather than absorption, these colors can be startlingly brilliant. The Blue Morpho butterflies of South and Central America are visible from a quarter of a mile away, seeming almost to shine when sunlight penetrates the tropical forest canopy and bounces off their wings.
Structural colors are just one example of how living organisms manipulate and channel light using delicately arranged micro- and nanostructures. These biological designs offer inspiration to engineers seeking to control light in optical technologies, and could lead to more brilliant visual displays, new chemical sensors, and better storage, transmission and processing of information. To make effective use of such tricks, we need to understand how nature creates and deploys these tiny optical structures; indeed, we must learn a new language of color production and mixing.
Rather little is known about how many of these biological structures are put together, how they evolved, and how evolution has made creative use of the color and light effects they offer. But one thing is clear; nature doesn’t have the sophisticated patterning technologies, such as drilling with electron beams, that microengineers can use to laboriously carve such structures from solid blocks. Ingenuity is used instead of finesse; these biological structures must make themselves from the component parts.
If we can master that art, we might develop new, cheap technologies to make such things as materials that change color or appearance, like the camouflage skins of some fish and squid, or fibres that guide and channel light with virtually no leakage, or chemically controlled light shutters. Here I look at some of nature’s tricks for turning structure into color; and the ways they are being exploited in artificial materials and devices (Ball, 2012).
Layers
Although the ridges seen by Hooke on butterfly wing scales do scatter light, the bright colors of the reflected light generally come from invisible structures beneath the surface. In the natural world, they offer a robust way of generating color that is not hostage to the fate of delicate, light-sensitive organic pigments.
The colored scales and feathers of birds, fish and butterflies typically contain organized microscopic layers or rods of a dense light-scattering material embedded in a matrix of a different substance. Because the distance between the scatterers is roughly the same as the wavelengths of visible light, the stacks cause the wave phenomenon of diffraction, in which reflected waves interfere with one another. Depending on the angle of reflection, light rays of a certain wavelength interfere constructively when they bounce off successive layers in the stack, boosting the corresponding color in the reflected light (Vukusic and Sambles, 2003; Vukusic, 2004; Wolpert, 2009). It is much the same process that elicits the chromatic spectrum in light glancing off a tilted CD.
In butterfly wing scales the reflecting stacks are made of cuticle; a hard material containing the natural polymer chitin, separated by air-filled voids. In bird feathers, the stacks are platelets or rods of the dark pigment melanin; sometimes hollow, as in the Black Inca hummingbird, Coeligena prunellei, embedded in keratin, the protein from which our hair and fingernails are made (Figure 2). Analogous diffraction gratings made from alternating ultrathin layers of two materials are widely used in optical technologies to select and reflect light of a single color. For example, mirrors made from multiple layers of semiconductors are used as reflectors and color filters in devices ranging from astronomical telescopes to solid-state lasers and spectrometers.
Figure 2. The iridescent blues and greens in the feathers of hummingbirds such as this Black Inca (left; part of blue iridescence highlighted with white box) are created by platelets of melanin pigment punctuated with air holes (right), which act as a photonic crystal to reflect light of a particular wavelength. K=keratin, A=air, M=melanin. (From Shawkey et al., 2009)
The male bird of paradise Lawes’ parotia (Parotia lawesii) has a particularly neat twist on this trick (Figure 3). The barbules (hair-like structures on the feather barbs) of its breast feathers contain layers of melanin spaced at a distance that creates bright orange-yellow reflection. But, as Stavenga and colleagues have recently discovered, each barbule has a V-shaped or boomerang cross-section, with sloping surfaces that also act as reflectors of blue light (Stavenga et al., 2011). Slight movements of the feathers during the bird’s courtship ritual can switch the color abruptly between yellow-orange and blue-green; guaranteed to catch a female’s eye. Stavenga suspects that technologists will want to use this trick for producing dramatic chromatic shifts. “I suspect the fashion or automobile industries will in due time make bent structures or flakes that will exploit these angular color changes”, he says.
Figure 3. A striking color change in the feathers of the male Lawes’ parotia, from yellow-orange (a) to blue-green (b), is caused by the presence of two mirror-like reflectors in the feather barbules (c): layers of melanin rods reflect yellow light, while the sloping faces of the boomerang-shaped barbule cross-section reflect blue at oblique angles. Scale bar in b: 1 cm. (From Stavenga et al., 2011)
Christmas Trees
The butterflies Morpho didius and Morpho rhetenor obtain their dazzling blue color not from simple multilayer’s but from more complex nanostructures in the wing scales: arrays of ornate chitin ‘Christmas Trees’ that sprout at the surface (Vukusic & Sambles, 2003) (Figure 4). Each ‘tree’ presents a stack of disk-like layers to the incoming light, which acts as another kind of diffraction grating. These arrays may reflect up to 80 percent of the incident blue light. And because they are not flat, they can reflect a single color over a range of viewing angles, somewhat reducing the iridescence; organisms don’t always want to change color or get dimmer when seen from different directions.
Figure 4. The butterfly Morpho didius (left) obtains its dazzling blue color from delicate ‘Christmas Tree’ light-scattering structures (right), made from chitin, that sprout within the wing scales. (Left, courtesy of Peter Vukusic. Right (micrograph) from Vukusic and Sambles, 2003.)
The precise color reflected depends on the refractive index contrast between the nanostructures and the surrounding medium. This is usually air, but as Robert Hooke observed, wetting such surfaces alters the refractive index contrast, and changes the color in a way that is closely linked to the wetting liquid’s refractive index. For that reason, artificial Morpho-like structures carved into solids using microlithographic techniques are being developed by researchers at GE Global Research in New York, in collaboration with others at the State University of New York at Albany and butterfly-wing expert Pete Vukusic at the University of Exeter in England, as color-change chemical sensors that can identify a range of different liquids (Potyrailo, 2011). These might find applications for sensing emissions at power plants, monitoring of food safety, and testing of water purity.
Reflecting Bowls
The bright green color of the Emerald Swallowtail butterfly (Papilio palinurus), found widely in southeast Asia, is not produced by green light at all. The wing scales are covered with a honeycomb array of tiny bowl-shaped depressions just a few micrometers across, lined with alternating layers of chitin cuticle and air which act as reflective mirrors. Light bouncing off the bottoms of the bowls is preferentially reflected in the yellow part of the spectrum. But from the sides it is reflected twice before bouncing back, and this selects blue. Our eyes can’t resolve these yellow spots and blue rings, which merge to create the perception of green (Vukusic & Sambles, 2003).
Figure 5. The green of the Emerald Swallowtail butterfly (left) comes from the optical mixing of blue and yellow reflections from tiny bowl-like depressions in the wing scales (right). (Right figure, courtesy of Christopher Summers, Georgia Institute of Technology)
This way of making color has been copied by Summers and coworkers (Crne et al., 2011). To create the tiny bowls, they let water vapour condense as microscopic droplets, called breath figures, on the surface of a polymer dissolved in a volatile solvent. The solvent gradually evaporates to form a solid polymer film, while the water droplets pack together on the surface of the drying solution much like greengrocers’ oranges and apples in crates, sinking into the setting film to imprint an array of holes. By pulling off the top part of the film, Summers and colleagues were left with a surface with hemispherical bowl-like dimples. They then used this structure as a template on which they deposited alternating thin layers of titania and alumina to make a multilayer reflector, like that lining the bowls of the butterfly wing scales (Figure 6).
Figure 6. An artificial micro-structured surface that mimics the green color of the Emerald Swallowtail. Scale bar: 5 µm. (Courtesy of Christopher Summers, Georgia Institute of Technology)
Because each reflection changes the polarization of the light, under crossed polarizing filters the yellow light bouncing back from a single reflection at the bowl centers disappears, while the twice-reflected blue-green light from the rims remains. This could offer a distinctive authentification mark on bank and credit cards. Apparently just a simple green reflective coating, such a material would in fact carry a hidden polarized signature in the reflected blue and yellow light that would be hard to counterfeit. But Summers’ collaborator Mohan Srinivasarao admits that the main reason for seeking to replicate the butterfly’s green color was that “it’s beautiful in its own right”.
Ordered Nanosponges
Scattering by regular arrays of microscopic objects can, for some arrangements, totally exclude light within a particular band of wavelengths, called the photonic band gap (Vukusic, 2004). These so-called photonic crystals occur naturally, for example, in opal, a biogenic form of silica in which the scatterers are tiny mineral spheres. Artificial photonic crystals can be used to confine light within narrow channels, creating waveguides that might be deployed to guide light around on silicon chips for optical information technology.
Nature has already got there first. Under the electron microscope, the wing scales the Emerald Patched Cattleheart Butterfly (Parides sesostris) display zigzagging, herring-bone arrays: patches of an orderly sponge made from chitin with holes a hundred nanometers or so across. Each patch is a photonic crystal seen from a different alignment. Stavenga and Michielsen have found that these labyrinths in the wing-scales of P. sesostris and some species of papilionid and lycaenid butterflies have a structure known to mathematicians as a gyroid (Michielsen & Stavenga, 2008). In P. sesostris the structure has a photonic band gap that enables it to reflect light within the green part of the spectrum over a wide range of incident angles (Figure 7). Some weevils and other beetles also derive their iridescent color from three-dimensional photonic crystals made of chitin.
Figure 7. The wing scales of P. sesostris (top left, and close-up, top right) contain photonic crystals of chitin (bottom, middle and right) Scale bars: left, 100 µm; middle, 2 µm; right, 2 µm. (Bottom figure, from Saranathan et al., 2010)
Richard Prum and coworkers have figured out how these photonic crystals grow (Saranathan et al., 2010). The molecules in the soft membranes that template the deposition of chitin during wing-scale growth become spontaneously organized into the ‘crystalline sponge’. Biological membranes are made up of long, tadpole-like molecules called lipids, which have a water-soluble head and an oily tail. To shield the tails from water, they cluster side by side into sheets with the heads pointing outwards; the sheets then sit back to back in bilayer membranes. Pores in these membrane induce curvature, partly exposing the lipid tails and therefore incurring a cost in energy. For this reason, the pores in effect repel one another, and this can force them to become arranged in a regular way, an equal distance apart. Periodic membrane structures have been found in the cells of many different organisms, from bacteria to rats (Hyde et al., 1997).
In P. sesostris wing-scale progenitor cells, the outer ‘plasma membrane’ and the folded membrane of the inner compartments called the endoplasmic reticulum, where lipids and other molecules are made, come together to form a so-called double-gyroid structure (Figure 8, left), in which two interweaving sets of channels divide up space into three networks that interpenetrate, but are isolated from one another. One of these is then filled with chitin, which hardens into a robust form while the cell dies and the rest of the material is degraded, leaving behind the single gyroid phase (Saranathan et al., 2010).
It has been suggested that these natural nanostructures might be used as the templates for making artificial ones, for example, by filling the empty space around the chitin with a polymer or an inorganic solid, and then dissolving away the chitin (Saranathan et al., 2010). But it is also possible to mimic the structures from scratch. For instance, artificial bilayer membranes made from lipid-like molecules called surfactants will also form orderly sponges, and so will so-called block copolymers, in which the chain-like molecules consist of two stretches with different chemical composition (Hyde et al., 1997). Ulrich Wiesner and coworkers (Stefik et al., 2012) have mixed liquid block copolymers with nanoparticles of niobium and titanium oxide, and let the polymers form into gyroid and other ordered ‘nanosponge’ structures that usher the nanoparticles into the same arrays. When this composite is heated, the polymer is burnt away while the mineral nanoparticles coalesce into continuous networks (Figure 8, center).
These porous solids could find a wide range of uses. Thin porous films of titanium dioxide nanoparticles coated in light-absorbing dyes are already used in low-cost solar cells. These orderly gyroid networks can offer improvements, partly because the solid material through which light-excited electrons are harvested is continuously connected rather than relying on random electrical contacts between nanoparticles. And the researchers have calculated that double-gyroid nanosponges made from metals such as silver or aluminum, which might similarly be assembled from nanoparticles guided by block copolymers, could have the weird property of a negative refractive index, meaning that they would bend light ‘the wrong way’ (Hur et al., 2011). Such materials could be used to make so-called superlenses for optical microscopes that can image objects smaller than the wavelength of light; something that isn’t possible with conventional lenses.
Inspired by the butterfly structures, Mark Turner and colleagues (Turner et al., 2011) have used laser beams to ‘write’ these intricate three-dimensional photonic crystals directly into a commercial light-polymerizable ‘photoresist’ material (Figure 8, right). Being somewhat ‘scaled-up’ versions of the natural nanostructures, these had photonic band gaps in the infrared part of the spectrum. Current telecommunications operates mostly at infrared wavelengths, and these structures could find uses there; some, for example, have a corkscrew lattice that make them respond differently to circularly polarized light with a left- or right-handed twist.
Figure 8. The gyroid phase (left), and structures mimicking the ‘butterfly gyroid’: (middle) a network of titania organized by self-assembly of a block copolymer, and (right) a larger-scale lattice made by setting a light-sensitive polymer with laser beams (scale bar: 10 µm). (Left figure, courtesy of Matthias Weber, Indiana University. Middle figure, from Stefik et al., 2012. Right figure, from Turner et al., 2011)
Photonic Crystal Fibers
The spines of some marine polychaete worms, such as Aphrodita (the sea mouse) and Pherusa, are tubular structures containing hexagonally packed hollow cylindrical channels a few hundred nanometers across and made from chitin. These arrays act as two-dimensional photonic crystals that reflect light strongly in the long-wavelength part of the spectrum, which gives the Aphrodite spine a deep, iridescent red color (Figure 9) (Parker et al., 2001; Trzeciak & Vukusic, 2009).
Figure 9. The tiny spines of polychaete worms such as the sea mouse (Polychaeta: Aphroditidae; top left) are natural photonic crystals. Seen close up in cross section, they consist of regularly packed hollow channels with walls of chitin. Middle left: cross-section from Pherusa (scale bar: 2 µm); center: side view of channels from Aphrodita; right: the red color of light passing through a spine of Aphrodita. Artificial photonic fibres like this can easily be made by heating and drawing out bundles of glass capillaries (bottom). They can confine light within the ‘solid’ channels even around tight bends. (Note the solid ‘defect’ in the central channel.) (Top, middle center and middle right, courtesy of Andrew Parker, University of Oxford. Middle left, from Trzeciak & Vukusic, 2009. Bottom, from Russell, 2003)
It is not clear if the optical properties of the polychaete spines have any biological function. But there are certainly uses for such light-manipulating fibres in optical technology. For example, Philip Russell and collaborators (Russell, 2003) have made them by stacking glass capillaries into hexagonally packed bundles and drawing them out under heat into narrow fibers laced through with holes. If ‘defects’ are introduced into the array of tubular channels, either by including a wider capillary or a solid rod in the bundle, light can pass along the defect while being excluded from the photonic crystal, creating an optical fiber with a cladding that is essentially impermeable to light of wavelengths within the band gap. Photonic crystal fibers like this can guide light around tighter bends than is usually possible with conventional fibers, where the light is confined less reliably by internal reflection at the fibre surface. As a result, these fibers would work better for guiding light in tightly confined spaces, such as on optical microchips. And because photonic crystal fibers are in general less ‘leaky’ than conventional ones, they could be replace them in optical telecommunications networks, requiring less power, and obviating the need for amplifiers to boost signals sent over long distances.
Disordered Nanosponges
The splendid blue and green plumage of many birds, while also being physical rather than pigmented colors, lacks the iridescence of the hummingbird or the peacock. Instead, they have the same color viewed from any angle. They scatter light from sponge-like keratin nanostructures; but because these structures are disordered, the scattering is diffuse, like the blue of the sky, rather than mirror-like and iridescent (Dufresne et al., 2009).
In the blue-and-yellow macaw, Ara ararauna, (Figure 10), and the black-capped kingfisher Halcyon pileata, the empty spaces in the keratin matrix of the feather barbs form tortuous channels about 100 nm wide. A similar random network of filaments in the cuticle of the Cyphochilus beetle gives it a dazzlingly bright white shell. In some other birds, such as the blue-crowned manakin, Lepidothrix coronata, the air holes are instead little spherical bubbles connected by tiny cavities.
Figure 10. The blue feathers of the blue-and-yellow macaw contain sponge-like labyrinths of air and keratin (bottom left), which scatter blue light strongly in all directions. Some other feathers derive similar colors from spherical ‘bubble-like’ air holes in the keratin matrix (bottom right). Scale bars: 500 nm. (Bottom figure, from Dufresne et al., 2009)
It is believed that both of these structures are formed as keratin separates out spontaneously from the fluid cytoplasm of feather-forming cells, like oil from water (Dufresne et al., 2009). In liquid mixtures, such as solidifying molten metal alloys or polymers, such phase separation creates different structures in different conditions. If the mixture is intrinsically unstable, the components separate into disorderly, interwoven channels in a process called spinodal decomposition. But if the mixture is metastable (provisionally stable), like water supersaturated with dissolved gas, then the separating phase will form discrete blobs or bubbles that grow from very tiny ‘seeds’ or nuclei. Prum thinks that either of these processes may happen as bird feathers develop, and that birds have evolved a way of controlling the rate of keratin phase separation so that they can arrest the nanostructure at a certain size. Once the cells have died and dried, this size determines the wavelength of scattered light, and thus the feather’s color.
This kind of diffuse light-scattering has been used for centuries as a way of making colors in technology. In milk, microscopic droplets of fat with a wide range of sizes cause scattering of all visible wavelengths, and give the liquid its opaque whiteness. Michael Faraday discovered in the nineteenth century that light scattering from nanoscale particles of gold suspended in water can create a deep reddish-purple color with a precise hue that depends on the size of the particles. Glassmakers had been using alchemical recipes to precipitate nanoscale gold particles in molten silica to make ruby glass ever since ancient times.
Today, engineers are looking at how these random networks and particle arrays can give rise to strongly colored and high-opacity materials. Pete Vukusic and colleagues (Hallam et al., 2009) have mimicked the cuticle of Cyphochilus beetles in random porous networks made from interconnected filaments of the minerals calcium carbonate and titanium dioxide mixed with a polymer and oil liquid binders and left to dry. Guided by the size and density of filaments in the beetle shell, they were able to make thin coatings with brilliant whiteness. Meanwhile Prum, his colleague Eric Dufresne and their coworkers at Yale University (Forster et al., 2010) have mimicked the disordered sponges of bird feathers by creating films of randomly packed microscopic polymer beads, which have blue-green colors (Figure 11).
Figure 11. This thin film of randomly arrayed polymer microspheres mimics the keratin matrix in the blue feathers of the blue-crowned manakin. (From Forster et al., 2010)
Color Change
One of the most enviable optical tricks in nature is to produce reversible color changes. The reflective, protean colors in the skins of squid such as the Loligidinae family are produced by a protein called reflectin, arranged into plate-like stacks in cells called iridophores, which again act as color-selective reflectors (Figure 12). The color changes are thought to be involved in both camouflage and communication between squid for mating and displays of aggression.
Figure 12. Stacked plates of the reflectin protein (left) in iridophore cells (center) create tunable reflective colors in squid (right). (Center figure, courtesy of Daniel Morse, University of California at Santa Barbara)
Daniel Morse and colleagues have recently figured out how the color changes of iridophores are achieved (Tao et al., 2010). The reflectin proteins crumple up into nanoparticles, which pack together into dense arrays that make up the flat layers. These layers are sandwiched between deep folds of the cell membrane. The color change can be triggered by neurotransmitter lipid molecules called acetylcholine, which activate a biochemical process that fixes electrically charged phosphate chemical groups onto the reflectin protein. These groups largely neutralize the proteins’ intrinsic charge and allow them to pack more closely together, increasing the reflectivity of the layers. At the same time, this compaction squeezes water from between the protein particles and out of the cell, and enables the reflectin layers to sit closer together.
Morse and colleagues (Holt et al., 2010) think that it should be possible to copy some of these tricks in optical devices, perhaps even using reflectins themselves. They have inserted the gene encoding a reflectin protein from the long-finned squid Loligo pealeii into Escherichia colibacteria. When expressed, the protein spontaneously collapses into nanoparticles (Tao et al., 2010). The size of these particles can be tuned by controlling the interactions between charged groups on the proteins using salt. Held between stacks of permeable membranes, these materials might therefore swell and contract, altering the reflected wavelengths, in response to chemical triggers. Morse and colleagues have also taken inspiration from reflectins to develop a light switch based on a wholly synthetic light-sensitive polymer. They use an electric field both to change the refractive index of the polymer and to pull salt into the polymer film to swell it. As with iridophores, this combination of effects alters the material’s response to light dramatically, switching it from transparent to opaque; all without moving parts or high-tech manufacturing methods. The team are currently working with Raytheon Vision Systems, an optics company in Goleta, California, to use this system in fast shutters for infrared cameras.
The Art and Science of Natural Color Mixing
Many of the optical effects found in nature are not purely due to structural colors, but arise from their combination with absorbing pigments (Shawkey et al., 2009). In squid, a thin pigment layer above the reflective layer acts as a filter that can modify the appearance, for example, making it mottled; reflective and absorbing to different degrees in different places. In bird feathers, the physical colors resulting from melanin nanostructures embedded in a keratin protein matrix can be tuned by light-absorbing filters of pigments, such as carotenoids, which absorb red and yellow light. The characteristic green plumage of parrots seems to be produced by laying a yellow pigment over a blue reflective layer of melanin and keratin (Figure 13). And the purple wing tips of Purple Tip butterflies come from red pigments beneath a blue iridescent surface.
Figure 13. Green is a characteristic color of parrots, but their plumage contains no green pigment, nor is it purely a structural color. Rather, it results from ‘structural blue’ overlaid with a filter of yellow pigment.
Chameleons display perhaps the most advanced mastery of these mixing tricks. Their spectacular color changes are produced by three separate systems for modifying the reflected light, stacked one atop the other. The first layer consists of cells containing red and yellow light-absorbing pigment particles, the location of which within the cell determines the color intensity. Below these are iridophores like those of squid, from which blue and white light may be selectively reflected by crystalline layers of the molecule guanine (also a component of DNA). Finally there is a layer of cells containing the dark pigment melanin, which act like the colored ‘ground’ layers of Old Master paintings to modify the reflection of light that penetrates through the first two layers. This combination of reflection and absorption enables the chameleon to adapt its skin color across a wide, albeit species-specific, range to signal warning, for mating displays, and for camouflage (Forbes, 2009).
How pigments alter and adjust the reflected light in such cases is still imperfectly understood. One problem is that the combinations are so diverse; more than 20 different arrangements of melanin, keratin and air have been identified in the plumage of birds. Moreover, melanin is itself a light absorber, creating colors ranging from yellow to black. The bright white markings on the blue wings of the Morpho cypris butterfly are produced by simply removing the melanin from reflective multilayer structures; the mirrors remain, but the pigments do not.
In such ways, evolution has made creative use of the limited range of materials at its disposal to generate a riot of profuse coloration and markings. A better understanding of how this is achieved could give painters and visual artists access to entirely new ways of making colors based on iridescent and pearlescent pigments, whose use has so far been largely restricted to less sophisticated applications in the automobile and cosmetic industries (Schenk & Parker, 2011).
Painter Franziska Schenk has been exploring the mixing of structural and pigmented color during her stay as artist-in-residence in the Department of Biosciences at the University of Birmingham in the UK (Schenk, 2009). With iridescent particles, says Schenk, “the established methods of easel painting no longer apply. Their conversion to painting requires something truly innovative.”
Schenk used iridescent particles to reproduce the starting blue of the Morpho wing in a series of paintings that change color when lit or viewed from different angles (Figure 14). The background color on which the particles are placed is central to the effect. On white, the light not reflected from the blue particles passes through and bounces off the base. This means that when not seen face-on, the blue quickly fades and is replaced by a muted yellow. But on a black background, all non-blue light is absorbed, and the blue is more pure and intense.
Figure 14. Painting of a Morpho butterfly wing by Franziska Schenk, using blue pearlescent pigments. The color changes depending on the angle of illumination, as well as on the nature of the background color. (Courtesy of Franziska Schenk)
Although the brilliance of these colors doesn’t approach that of butterfly wings, it takes advantage of recent improvements in synthetic pearlescent particles. The first of these were made by coating mica flakes with multilayers of metal oxides to generate the diffraction grating. But because the mica surfaces were not perfectly smooth and the grain sizes varied, there was always a range in the precise colors and intensities of the particles. Schenk has used pigments in which the mica substrate is replaced by a transparent borosilicate glass, which is smoother and gives a purer hue. She believes that “iridescent technology is destined to introduce a previously unimaginable level of intensity and depth, thus adding beauty, luster and a dynamic dimension to art”. Schenk’s Studies of Cuttlefish (Figure 15) is a painting that uses iridescent flakes mixed with beads and wax.
Figure 15. “Studies of Cuttlefish” by Franziska Schenk, using iridescent flakes mixed with beads and wax. (Courtesy of Franziska Schenk)
Another series of cuttlefish, “Mantle of Many Colours” (Figure 16), was made with iridescent paint that differs in appearance depending on the conditions and angle of lighting, which results in a compelling chameleon effect that traditional paints simply cannot create. The colors change from greens to purples as the viewing angle shifts. “Still images, together with any attempt to verbally describe the effect, are pretty limiting”, Schenk admits; you have to see these things in the flesh to appreciate their full impact.
Figure 16. “Mantle of Many Colours” by Franziska Schenk, which uses iridescent paint, as seen from different angles. (Courtesy of Franziska Schenk)
Conclusion
“Every day you play with the light of the universe”, wrote the Chilean poet Pablo Neruda, but he had no idea how literally true this would become. Our technologies for transmitting, manipulating and displaying information, whether for work or play, depend increasingly on our ability to control light; to harness and transform color. Some of nature’s most stunning sights depend on such a facility too, and often they show us that beauty can be inextricably linked to utility. We are impressed by plumage, by markings and animal displays, that are specifically designed by evolution to make such an impression. And nature has found ways to make this chromatic exuberance robust, changeable, responsive, and cheap and reliable to manufacture. In shaping color without the chemical contingency of pigments, there seems to be little we can dream up that nature has not already anticipated, exploiting its capacity to fashion intricate fabrics and structures on the tiniest scales. We can only learn, and admire.
References
Note: The current article is an extended version of Ball P (2012), “Nature’s color tricks”, Sci. Am. 306(5), 74-79.
Crne M, Sharma V, Blair J, Park J O, Summers C J & Srinivasaro M (2011), “Biomimicry of optical microsctructures of Papilio palinurus”, Europhys. Lett. 93, 14001.
Dufresne E R, Noh H, Saranathan V, Mochrie S G J, Cao H & Prum R O (2009), “Self-assembly of amorphous biophotonic nanostructures by phase separation”, Soft Matter 5, 1792-1795.
Forbes P (2009), Dazzled and Deceived: Mimicry and Camouflage. Yale University Press, New Haven.
Forster J D, Noh H, Liew S F, Saranathan V, Schrenk C F, Yang L, Park J-G, Prum R O, Mochrie S G J, O’Hern C S, Cao H & Dufresne E R (2010), “Biomimetic isotropic nanostructures for structural coloration”, Adv. Mater. 22, 2939-2944.
Hallam B T, Hiorns A G & Vukusic P (2009), “Developing optical efficiency through optimized coating structure: biomimetic inspiration from white beetles”, Appl. Opt. 48, 3243-3249.
Holt A L, Wehner J G A, Hampp A & Morse D E (2010), “Plastic transmissive infrared electrochromic devices”, Macromol. Chem. Phys. 211, 1701-1707.
Hooke R (1665), Micrographia. Reprinted by BiblioBazaar, p. 294. Charleston, South Carolina, 2007.
Hur K, Francescato Y, Giannini V, Maier S A, Hennig R G & Wiesner U (2011), “Three-dimensionally isotropic negative refractive index materials from block copolymer self-assembled chiral gyroid networks”, Angew. Chem. Int. Ed. 50, 11985-11989.
Hyde S, Blum Z, Landh T, Lidin S, Ninham B W, Andersson S & Larsson K (1997), The Language of Shape. Elsevier, Amsterdam.
Michielsen K and Stavenga D G (2008), “Gyroid cuticular structures in butterfly wing scales: biological photonic crystals”, J. R. Soc. Interface 5, 85-94.
Parker A R, McPhedran R C, McKenzie D R, Botten L C & Nicorovici N-A P (2001), “Aphrodite’s iridescence”, Nature 409, 36-37.
Potyrailo R A (2011), “Bio-inspired device offers new model for vapor sensing”, SPIE Newsroom, 10.1117/2.1201103.003568.
Russell P (2003), “Photonic crystal fibers”, Science 299, 358-362.
Saranathan V, Osuji C O, Mochrie S G J, Noh H, Narayanan S, Sandy A, Dufresne E R & Prum R O (2010), “Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales”, Proc. Natl Acad. Sci. USA 107, 11676-11681.
Schenk F (2009), “Nature’s fluctuating colour captured on canvas?”, Int. J. Design & Nature and Ecodynamics 4(3), 1-11.
Schenk F & Parker A (2011), “Iridescent color: from nature to the painter’s palette”, Leonardo 4(2) [no page numbers].
Shawkey M D, Morehouse N I & Vukusic, P (2009), “A protean palette: colour materials and mixing in birds and butterfiles”, J. R. Soc. Interface 6, S221-S231.
Stavenga D G, Leertouwer H L, Marshall N J & Osorio D (2011), “Dramatic colour changes in a bird of paradise caused by uniquely structured breast feather barbules”, Proc. R. Soc. B 278, 2098-2104.
Stefik M, Wang S, Hovden R, Sai H, Tate M W, Muller D A, Steiner U, Gruner S M & Wiesner U (2012), “Networked and chiral nanocomposites from ABC triblock terpolymer coassembly with transition metal oxide nanoparticles”, J. Mater. Chem. 22, 1078-1087.
Tao A R, DeMartini D G, Izumi M, Sweeney A M, Holt A L & Morse D E (2010), “The role of protein assembly in dynamically tunable bio-optical tissues”, Biomaterials 31, 793-801.
Turner M D, Schröder-Turk G E & Gu M (2011), “Fabrication and characterization of three-dimensional biomimetic chiral composites”, Opt. Express 19(10), 10001-10008.
Vukusic P and Sambles J R (2003), “Photonic structures in biology”, Nature 424, 852-855.
Vukusic P (2004), “Natural photonics”, Physics World 17 (2), 35-39.
Trzeciak T M & Vukusic P (2009), “Photonic crystal fiber in the polychaete worm Pherusa sp.”, Phys. Rev. E 80, 061908.
Wolpert H D (2009), “Optical filters in nature”, Optics and Photonics News 20(2), 22-27.
05/30/12
Technology of Nanostructures
Colloidal Self Assembly for fabrication of Photonic nanostructures including
Colloidal crystals
Composite and Inverse Opals
Photonic Glasses
Applications
Displays
Optical Devices
Photochemistry
Biological Sensors
Source: Self-assembled colloidal structures for photonics
Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies
Nicolas Vogel, Stefanie Utech, Grant T. England, Tanya Shirman, Katherine R. Phillips, Natalie Koay, Ian B. Burgess, Mathias Kolle, David A. Weitz, and Joanna Aizenberg
PNAS September 1, 2015 112 (35) 10845-10850; first published August 19, 2015;
College of Information Science and Engineering, Northeastern University, Shenyang 110004, China2College of Information & Control Engineering, Shenyang Jianzhu University, Shenyang 110168, China3Melbourne Centre for Nanofabrication, Clayton, Victoria 3168, Australia*Authors to whom correspondence should be addressed.
Viburnum tinus Fruits Use Lipids to Produce Metallic Blue Structural Color
Rox Middleton,1,8,10 Miranda Sinnott-Armstrong,2,9,10 Yu Ogawa,3 Gianni Jacucci,1 Edwige Moyroud,4,5 Paula J. Rudall,6 Chrissie Prychid,6 Maria Conejero,6 Beverley J. Glover,7 Michael J. Donoghue,2 and Silvia Vignolini
HIGHLY DIFFRACTING, COLORSHIFTING, POLYMERIZED CRYSTALLINE COLLODAL ARRAYS OF HIGHILY CHARGED POLYMER SPHERES, PAINTS AND COATINGS AND PROCESSES FOR MAKING THE SAME
Structural color and its interaction with other color-producingelements: perspectives from spiders
Bor-Kai Hsiung*, Todd A Blackledge, and Matthew D Shawkey Department of Biology and Integrated Bioscience Program, The University of Akron, Akron, Ohio
Ming Xiao,1* Ziying Hu,2,3* Zhao Wang,4 Yiwen Li,5 Alejandro Diaz Tormo,6 Nicolas Le Thomas,6 Boxiang Wang,7 Nathan C. Gianneschi,2,3,4† Matthew D. Shawkey,8,9† Ali Dhinojwala
“Guanigma”: The Revised Structure of Biogenic Anhydrous Guanine
Anna Hirsch,† Dvir Gur,‡ Iryna Polishchuk,§ Davide Levy,§ Boaz Pokroy,§ Aurora J. Cruz-Cabeza,∥ Lia Addadi,*,‡ Leeor Kronik,*,† and Leslie Leiserowitz*,†
Bio-Inspired Structural Colors Produced via Self-Assembly of Synthetic Melanin Nanoparticles
Ming Xiao,†,^ Yiwen Li,‡,^ Michael C. Allen,§ Dimitri D. Deheyn,§ Xiujun Yue,‡ Jiuzhou Zhao,† Nathan C. Gianneschi,*,‡ Matthew D. Shawkey,*, and Ali Dhinojwala
Bright and Vivid Diffractive-Plasmonic Structural Colors
Emerson G. Melo,†,‡,§ Ana L. A. Ribeiro,†,‡ Rodrigo S. Benevides,†,‡ Antonio A. G. V. Zuben,†,‡ Marcos V. P. Santos,† Alexandre A. Silva,¶ Gustavo S. Wiederhecker,†,‡ and Thiago P. M. Alegre
The dominant class of Pearlescent Pigments is represented by natural mica coated with thin films of different metal oxides[2]. Mica based pigments were firstly developed in the 1970s and got accelerated until 1990s when multilayer systems on mica were successfully realized. Natural muscovite mica is a rather inexpensive crystal and it can be easily cleaved to thinner flakes of typically 250 nm. These advantages made mica-based pigments quickly monopolize the special effect pigment market, until reaching 90% of the whole market. This pigment is easily produced by the deposition of metal oxide layers on the mica surface[2]. TiO2 or iron oxide covered mica pigments can be easily produced with a high thickness control, but sometimes they show limited optical properties[10]. Mica-based pigments with multilayers show pronounced angle dependence, but they are heavier respect to other pearlescent pigment types, thus leading to a higher pigment content required to reach a certain colour strength[11][12][13]. There are many emerging substrate-based pigments, different from the ones based on mica substrates, that show interesting optical properties. Pigments based on silica flakes (SiO2) are easily produced in a very controlled and uniform thickness by the web-coating process[5]. The thickness of silica flakes is in the order of 400 nm, comparable to that of mica particles, and it can be tailored be so narrow to become itself an optical layer. These pigments allow obtaining a high chromatic strength and special colour travel effects, useful for automotive applications, decorative plastics and security inks[14]. Alumina (Al2O3) based pigments represent another type of emerging pearlescent pigment[5]. This pigment type has got strong pearlescent effect respect to mica-based pigments mainly due to its high aspect ratio and narrow thickness distribution, as it happens for silica-based pigments. In addition to that, alumina-based pigments exhibit unique crystal-like effect (sparkle effect), mainly due to their smooth surface and chemical purity, thus being interesting for high-duty decorative purposes, such as car paints[2]. The study recently published by our group study the effect of the addition of alumina-based pigments on the durability of powder coatings. The pearlescent pigments taken under consideration were supplied by Merck S.p.A (Darmstadt, Germany). Figure 2 shows an SEM image of one of the pigments used in this study.
In order to be complete, it necessary to mention the presence of other substrate-based pigments such as pigments based on glass flakes and aluminium flakes. Pigments based on glass substrates play a minor role in the market because they are very thick and show limited optical properties, apart from special applications. Pigments base on aluminium flakes are produced via CVD processes and show an interesting angle-dependent colour changing, but the variety of colours available is limited to gold, orange and reddish metal-like colours.
Source: EFFECT PIGMENTS FOR INDUSTRY
Source: Photo-realistic Rendering of Metallic Car Paint from Image-Based Measurements
Source: QCV Korea
Source: QCV Korea
Source: QCV Korea
Source: QCV Korea
Source: CCM System for Metallic and Pearlescent Colors
Types of Metallic and Pearlescent Pigments
( Composition Based)
Source: Effect pigments—past, present and future
Effect pigments without a layer structure—substrate-free pigments
Metal effect pigments
Flakes or lamellae of
aluminum (“aluminum bronzes”)
copper
copper-zinc alloys (“gold bronzes”)
zinc
other metals
Natural pearl essence
Basic lead carbonate
Bismuth oxychloride
Micaceous iron oxide
Titanium dioxide flakes
Flaky organic pigments
Pigments based on liquid crystal polymers
Substrate based, Pearlescent Pigments, Layered
Mica Based
Alumina Based
Silica Flakes based
Glass Flakes Based
Iron Oxide Flakes Based
Graphite Flakes Based
Aluminum Flakes Based
Multilayer structures of the Fabry–Perot type
Structural arrangements consisting of alternating thin metal and dielectric layers can be used to achieve strong angle-dependent optical effects, e.g., in form of so-called optically variable pigments (OVP) [5,12]. Different color shifts can be produced by precisely controlled thickness of the multilayers. The metal layers consist in most cases of chromium (semitransparent absorber layers on the top and the bottom of a five-layer system) and of aluminum (opaque reflector layer in the center of the layer structure). The dielectric layers in between the chromium and aluminum layers consist mostly of magnesium fluoride or silicon dioxide. Such layer systems are the basis for an optical interference phenomenon called Fabry–Perot effect, which is different from interference effects of transparent layer systems because of the complete reflection of the light at the opaque reflector layer. Symmetrical arrangements of at least five layers are necessary to achieve strong color-shifting effects. In the case of pigments, only the five-layer systems play a role for practical use.
Effect pigments—past, present and future
Source: Industrial Inorganic Pigments / Edited by G. Buxbaum and G. Pfaff
Source: Industrial Inorganic Pigments / Edited by G. Buxbaum and G. Pfaff
Types based on Optics
Multiple Reflection
Refractive Pigments
Interference Pigments
Diffraction Pigments
Holographic Pigments
Source: Fascinating displays of colour Effect pigments – A successful interplay of chemistry and physics
Source: Ceramic Coatings for Pigments
Special Effects Pigments
( Luster Pigments)
Pearl luster pigments
Pearlescent pigments
Nacreous pigments
Interference pigments
Metal effect pigments.
All these pigments consist of small thin platelets that show strong lustrous effects when oriented in parallel alignment in application systems (e.g. in paints, plastics, printing inks, cosmetic formulations).
Sirio Pearl A4 Paper with Metallic Effect, 125 g, Ideal for Weddings, Christmas, Greeting Cards
Netuno x Sheets Pearlescent Azure Blue A4 210 x 297 mm Majestic Damask Blue
Netuno x Sheets Pearlescent Silver Paper DIN A4 210 x 297 mm Majestic Moonlight Silver
Sirio Pearl Red Fever 10 x Sheets of Pearlescent Red 300 g Paper DIN A4 210 x 297 mm Ideal for Weddings, Birthdays, Christmas, Invitations, Diplomas,…
10 Sheets of Mother of Pearl Gold 290 g Cardboard DIN A4 210 x 297 mm Cocktail Mai Tai, Ideal for Wedding, Birthday, Christmas, Invitations, Diplomas, Arts…
10 x A4 Frost White Pearlescent Shimmer Paper 120gsm Suitable for Inkjet and Laser Printers (PIA4-5)
A4 Pink Pearlescent Gold Card 300gsm Rose
Netuno x Sheets Pearlescent Dark Blue Paper DIN A4 210 x 297 mm Majestic Kings Blue
Nettuno Oltremare, 10 sheets, blue cardboard, 215 g, felt marked on both sides, with line structure, DIN A4, 210 x 297 mm, ideal for wedding, birthday,…
25 Sheets Light Green Coloured Card DIN A4 210 x 297 mm 210 g Sirio Colour Lime, Ideal for Weddings, Birthdays, Christmas, Invitations, Diplomas, Business Cards, Scrapbooking, Crafts and Decorating
10 x A4 Pearlescent Intense Shine Mellow Gold Paper 120 g/m² Double Sided For Inkjet and Laser Printers
A4 Paper Sea Blue Pearlescent Paper 100 g/m² for Inkjet and Laser Printers
20 Sheets A4 Maya Blue Pearlescent Paper 100gsm for Inkjet and Laser Printers
20 x A4 Printer Paper Damask Majestic Light Blue Double-sided Peregrina Pearl 120 g/m², suitable for inkjet and laser printers
10 x A4 Gold Peregrina Real Gold Pearlescent Effect Paper 120gsm Double Sided Suitable for Inkjet and Laser Printers
Cosmetics
Source: Merck KGaA
Black Color Pigments
Color Luster Pigments
Color Travel Pigments
Gold Pigments
Interference Pigments
Metallic Color Luster Pigments
Silverwhite Pigments
Automotive Paints
Automotive Coatings: Creating Excitement with Effect Pigments
By Cynthia Challener, CoatingsTech Contributing Writer
Regardless of the end-use application, special effect pigments provide a differentiated appearance. That is certainly true in the automotive industry, where they are used in coatings applied to both the interior and exterior of vehicles. Shifts in customer color and appearance preferences drive the use and development of effect pigments, as do developments in coatings technology and application processes. High sparkle finishes and intensely chromatic colors on car bodies and mirror-like finishes on interior components are increasing in popularity and driving the use of glass flakes, colored aluminums, and aluminum pigments with a much finer particle size. Pigments also need to provide the same appearance in coatings with thinner and/or fewer layers while exhibiting increased durability.
Creating a Unique Look
Coatings formulators work directly with pigment suppliers to develop and commercialize new specialty effect pigments to generate exciting color spaces that accentuate the bodylines of new vehicles. Effect pigments are the fastest growing segment of the high performance pigment market, and in 2015 were present in 70% and 65% of automotive colors for new builds in the Americas and Europe, respectively, according to Jane Harrington, manager of color styling with PPG Automotive OEM Coatings. “While neutral colors such as white, black, and silvers still dominate most of the automotive color palette, deep, rich, highly chromatic blues, greens, oranges, and reds have begun to find their place in the automotive world as well,” says Jason Kuhla, manager of technical service & product application with Silberline Manufacturing. “Special effect pigments that provide brilliance and ‘pop’ can help to create a look that stands out among the sea of color monotony, and appeal to those consumers who wish to stand apart from the crowd,” he adds. Allen Brown, advanced development and mastering manager in the Color and Material Design group of Ford Motor Company, agrees that while there will always be niche colors for special applications, overall there seems to be a balancing of colors to round out a complete selection, with a shift away from achromatic colors to a more sophisticated, balanced palette. For some applications, designers are seeking to create a value-added appearance by increasing the brilliance and reflectivity of metallic finishes while maintaining a smooth, non-sparkling appearance, according to Michael T. Venturini, global marketing manager, Coatings, Sun Chemical Performance Pigments.
Effect pigments are the fastest growing segment of the high performance pigment market, and in 2015 were present in 70% and 65% of automotive colors for new builds in the Americas and Europe . . . .
To achieve the desired appearance, most pigment flakes must be oriented in a specific manner within the coating. Their particle size also impacts the way they interact with light; larger particles provide more sparkle and iridescence, but the dimensions are limited to avoid impacts on gloss and other appearance properties. The industry is pushing the limits in this area, according to Paul Czornij, technical manager with the Color Excellence Group of BASF Coatings, and is seeking as much coarseness as the color can allow yet still providing a smooth and glossy look. The rheology of effect pigments, particularly in high solids, solvent-based systems, also influences their final appearance properties. On the other hand, there is a desire for smoother glass-like finishes, which has led to greater use of finer particle sizes to help deliver a quality liquid appearance in many colors, according to Brown. However, smoother finishes that give strong travel (bright face and dark side-tone) are difficult to achieve with electrostatic bell application (preferred for its greater transfer efficiency), which tends to make flakes stand up and give a more granular appearance, according to John Book, product line manager with Viavi. “Smaller particle sizes and size distributions also have a negative impact on color capability and metallic orientation, so such advances are far from simple,” asserts Frank Maimone, manager of pigment and color technology for the Color Development group of PPG Automotive OEM Coatings.
The shape of the vehicle has a significant impact on which effect pigments are used. For instance, fine/bright effect pigments that give coatings brightness with higher travel are preferred for vehicles that have a more interesting, free-flowing form, while for trucks, which are more slab-sided, coatings with more sparkle are frequently used, according to Jerry Koenigsmark,* who was manager of technical color design for PPG Automotive Coatings in North America. “For many of the new car designs targeting a younger consumer base, there is a push towards highly chromatic colors that employ colored aluminum pigments, mica pigments, glass flakes, and interference pigments,” says Kuhla. He also notes a shift in the wheel coatings market, where black is becoming more popular at the expense of traditional silvers.
For car interior trim parts, chrome-like coatings are used to create a value-added look and add haptic properties to simple plastic and other components. Auto parts and accessories (APA) also tend to be dominated by silvers, and many of these coatings contain pigments manufactured using physical vapor deposition (PVD) processes. In addition, many interior coatings are intended to provide attractive haptic properties. Because they are often single-layer systems, the effect pigments must have high resistance to body oils, perspiration, lotions, cosmetics, and other chemicals, according to Jörg Krames, vice president for global key account management with Eckart. He also notes, in these applications, liquid coatings compete with powder coatings and alternative technologies such as in-mold decoration with foils.
Finding Functional and Sustainable Solutions
Numerous other factors influence the choice of effect coatings beyond the appearance a designer wishes to create. In addition to provoking an emotional response in car buyers, effect pigments are often expected to serve multiple additional functions, according to Krames. The functional performance will be dictated by the type of coating and coating application systems. For external coatings, the compact application processes (primerless coating systems, three-coat/one-bake, integrated processes) widely used today on exterior car bodies involve the application of only a basecoat and topcoat over the e-coat. “Effect pigments in these systems must provide hiding power and exhibit high chemical-, moisture-, and UV-resistance properties in order to protect the e-coat,” he says. In addition, coating formulations now have higher pigment concentrations in smaller volumes, and the coating layers are either thinner or the flash times are eliminated. “Both scenarios have a negative impact on coating appearance and require extensive reformulation of coatings to meet end-use expectations,” notes Thomas A. Cook, global manager for color and process technologies with PPG Automotive OEM Coatings.
The trend towards thinner coatings has driven the development of new low-aspect-ratio effect pigment particles like colored, thin silver-dollar aluminum pigments that deliver brilliant metallic luster in high-chroma hues with good hiding and gloss. Generally, the use of smaller particle sizes will provide a smoother appearance with good gloss. However, to achieve the most chromatic colored effects and good flop behavior, manufacturers must consistently deliver highly optimized particle size distributions, comments Mike Crosby, market segment manager for BASF’s Global Automotive/OEM Pigments Business Unit. New lightweight substrates have surface-roughness and adhesion issues that also require coating reformulation, according to Bill Eibon, director of technology acquisitions for PPG Automotive OEM Coatings. On a positive note, Brown points out that ultra-smooth primers have helped to achieve a better glass-like appearance by creating a smoother base on which to paint.
Such integrated processes are just one response by the automotive industry to improve sustainability, reduce the use of hazardous materials and its carbon footprint, and meet increasing governmental regulatory requirements—all while ensuring outstanding value and consumer satisfaction, according to Czornij. “These imperatives are driving innovation and change, and even as formul
Note the transistors at the top of each colored pixel.
A spec comparison of common TFT types.
Higher currents are required for stable OLED panels, which a-Si falls short of.
LTPS is by far the most commonly used backplane technology, when you combine its use in LCD and AMOLED panels.
Smaller transistors allow for higher pixel densities
Higher resolution smartphones, such as the LG G3, are putting increasing demands on the transistor technology behind the scenes.
LTPS vs CBRITE spec comparison for use with OLED displays
Green
Cr3+ replacing Al3+ in octahedral site
Red/Green
Cr3+ replacing Al3+ in octahedral site
Red
Fe2+ replacing Mg2+ in 8- coordinate site
Peridot
T ourmaline
ab Weixia Zhang, 
b Ke Xuc and Yuanjin Zhao
