E-Paper Displays E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley Series in Display Technology
• Sprache: Englisch

Seitenzahl: 654

Veröffentlichungsjahr: 2022

Beschreibung

E-PAPER DISPLAYS An in-depth introduction to a promising technology, curated by one of its pioneering inventors Electronic paper (e-paper) has one of the most promising futures in technology. E-paper's potential is unlimited, as the displays require extremely low power and imitate the aesthetic of ink on the page. This allows e-paper devices to have a wider range of viewing angles than traditional LED products and are capable of being viewed in direct sunlight--and without any additional power. As a result, e-paper displays create less eye strain, have a greater flexibility in their use, and have the potential to be used in place of paper for billboard advertising, educational applications, and transport signage, and more. In E-Paper Displays, editor Bo-Ru Yang and his team of experts present a detailed view into the important technologies involved in e-paper displays, with a particlular emphasis on how this technology's unique properties make possible a wide range of personal and professional electronic products. As climate change makes efficient energy use more important than ever, e-paper can become an essential tool for future products on a large scale. As we rely more and more on technology, having lightweight devices with long battery life will become critical. This book provides engineers and innovators with an introduction to this important technology and shows new pathways for development. E-Paper Displays readers will also find: * The editor is one of the leading pioneers in this technology * Contributions from an international team of experts in e-paper technology * Descriptions of many advanced display types that rely on different principles than the widely used LCD and OLED types * Another innovative title from Wiley-SID (Society for Information Displays) series As we enter a new stage in our industrial development, E-Paper Displays is an essential reference for computer engineers and developers, as well as innovators and scientists, and their students.

Leseprobe

Table of Contents

Cover

Series Page

Title Page

Copyright Page

List of Contributors

Series Editor's Foreword

Editor's Preface

1 The Rise, and Fall, and Rise of Electronic Paper

1.1 Introduction

1.2 Why Electronic Paper?

1.3 Brightness, Color, and Resolution

1.4 Reflectivity and Viewing Angle

1.5 Translating Print‐on‐Paper into Electronic Paper

1.6 The Allure of Electronic Paper vs. the Practicality of LCDs

1.7 The Evolution of Electrophoretic Display‐Based Electronic Paper

1.8 Initial Wave of Electrophoretic Display Development

1.9 The Revival of EPDs

1.10 Developing a Commercial Display

1.11 Enhancing Brightness and Contrast

1.12 Microencapsulation Breakthrough

1.13 Image Retention

1.14 Active‐Matrix Compatibility

1.15 Electronic Book Products, and E Ink Merger

1.16 Summary

References

2 Fundamental Mechanisms of Electrophoretic Displays

2.1 General View of Electronic Ink Operation

2.2 Charging Mechanism with Inverse Micelle Dynamics

2.3 Drift and Diffusion of Charged Inverse Micelles

2.4 Motion of Charged Inverse Micelles Under External Field Driving

2.5 Stern Layer Formation

2.6 Charging Mechanism with Particles and Additives

2.7 Observations on a Single Particle

2.8 Rheological Effects During Driving

2.9 Bistability After Removing External Fields

2.10 Full Color E‐Paper

2.11 Conclusion

References

3 Driving Waveforms and Image Processing for Electrophoretic Displays

3.1 Driving Waveforms of EPDs

3.2 Image Processing

3.3 Advanced Driving Methods for Future E‐Papers

References

4 Fast‐Switching Mode with CLEARInk Structure

4.1 Introduction

4.2 CLEARink Display Optics

4.3 CLEARink Reflective Color Displays

4.4 Electrophoretic Displays with CLEARink Structure

4.5 CLEARink Device Architecture

4.6 Manufacturing and Supply Chain

Acknowledgments

References

5 Bistable Cholesteric Liquid Crystal Displays – Review and Writing Tablets

5.1 Introduction

5.2 Materials and Optical Properties

5.3 Image Creation Using Cholesteric Liquid Crystals

5.4 Applications

5.5 Writing Tablets

5.6 Conclusions

References

6 The Zenithal Bistable Display

6.1 Introduction

6.2 Operating Principles and Geometries

6.3 Grating Fabrication and Supply Chain

6.4 ZBD LCD Manufacturing Processes

6.5 Electrical Addressing

6.6 Optical Configurations

6.7 Novel Arrangements

6.8 Conclusions

Acknowledgments

References

7 Reflective LCD with Memory in Pixel Structure

7.1 Introduction

7.2 Memory in Pixel Technology and Its Super Low Power Operation

7.3 Sub‐Pixel Pattern to Show Gray Scale

7.4 Reflective LCD Optical Design

7.5 How to Show a Natural Image

7.6 Design Characteristics of Current Market‐Available Products and Their Super Low Power Operations

7.7 Summary of Power Consumption

7.8 Applications

7.9 Future Expectations

References

8 Optically Rewritable Liquid Crystal Display

8.1 Introduction

8.2 Photoalignment Technology

8.3 Flexible Optically Rewritable LCD

8.4 Dye‐Doped Optically Rewritable LCD

8.5 Conclusion

References

9 Electrowetting Displays

9.1 Overviews

9.2 Introduction

9.3 The Promise of Electrowetting Displays

9.4 History of Electrowetting Display Development

9.5 Electrowetting Cells

9.6 Capabilities for Black and White

9.7 Capabilities for Video and Color

9.8 Driving

9.9 Architectures

9.10 Manufacturing

9.11 Reliability

9.12 Failure Mechanisms

9.13 In Conclusion: Electrowetting Displays Have Reached Maturity

Acknowledgments

References

10 Electrochromic Display

10.1 Introduction

10.2 Structure of Electrochromic Display

10.3 EC Materials

10.4 Summary

References

11 Phase Change Material Displays

11.1 Introduction

11.2 Phase Change Materials and Devices

11.3 Strong Interference in Ultra‐Thin Absorbing Films

11.4 Potential for High Brightness, Low Power Color Reflective Displays

11.5 Solid‐State Reflective Displays (SRD

®

)

11.6 SRD Prototype – Progress and Performance

11.7 Other Approaches

11.8 Conclusions

Acknowledgments

References

12 Optical Measurements for E‐Paper Displays

12.1 Introduction

12.2 Fundamentals of Reflection

12.3 Reflection Measurements Set‐Ups

12.4 Display Image Quality Parameters

12.5 Temporal Parameters

12.6 Further Topics

12.7 Summary

Glossary incl. Abbreviations

References

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Summary of performance and other key factors for monochrome e‐Pap...

Chapter 2

Table 2.1 Properties of the five devices used for measurements.

Chapter 4

Table 4.1 White and dark state reflectance measured using the 5° to 30° Rin...

Table 4.2 Measured white state luminance and dark state luminance from stan...

Table 4.3 Power consumption estimates for a 9.7ʺ active matrix display.

Table 4.4 Estimated battery life of an eReader using a CLEARink type displa...

Chapter 6

Table 6.1 Operating parameters of a typical commercial ZBD display.

Chapter 7

Table 7.1 The area and the perimeter of LSB and MSB for “3 divided patterns...

Chapter 8

Table 8.1 Parameters of six liquid crystals.

Chapter 9

Table 9.1 Comparison of Lightness level and related luminance as given in S...

Chapter 11

Table 11.1 Comparison between PCM requirements for data storage and display...

List of Illustrations

Chapter 1

Figure 1.1 Arches 100% cotton rag paper. Scanning electron microscope image ...

Figure 1.2 CIELAB color system [4] John Wiley & Sons.

Figure 1.3 Color separation of an image into its CMYK components, and the fi...

Figure 1.4 Subtractive color mixing. Cyan and magenta overlap to make blue, ...

Figure 1.5 Examples of halftone dots of variable size, enabling grayscale [5...

Figure 1.6 Printed color generation through printed CMYK halftones [5] / Sli...

Figure 1.7 Color electrophoretic display using B/W particles and subpixel co...

Figure 1.8 Eight primary colors of ACeP by different pigment arrangements [1...

Figure 1.9 Prototypes of colored electrophoretic displays: the left image us...

Figure 1.10 Device reflectance vs contrast ratio for EPD at different device...

Figure 1.11 First EPD product for

point‐of‐purchase

(

POP

) use, a...

Figure 1.12 Demonstration of the first active‐matrix electrophoretic display...

Figure 1.13 SID 1998 Digest paper [80] Drzaic P, et.al, 1998 / John Wiley & ...

Figure 1.14 Microphotograph of Microcapsules in blue state [112].

Figure 1.15 Microphotograph of Microcapsules in white state. From SID 1997 D...

Figure 1.16 Largest capsules are on the order of 100 μm. From 1998 Digest pa...

Figure 1.17 Schematic cross‐section of microcapsules e‐Paper display.

Figure 1.18 Each square is 127 μm on a side. From Bouchard 2004 [88].

Figure 1.19 Lightness (L*) as a function of time (t). Lightness states were ...

Figure 1.20 Encapsulation construction with [98] / John Wiely & Sons, Inc.

Chapter 2

Figure 2.1 The ghosting phenomenon. (a) schematic optical trace of particles...

Figure 2.2 The general structure of a waveform for EPD driving.

Figure 2.3 Multi‐gray scale of E‐paper driven by PAM and PWM [1] / John Wile...

Figure 2.4 Schematic picture of the disproportionation mechanism of inverse ...

Figure 2.5 Five analytical regimes in

φ

and

λ

parameters space, re...

Figure 2.6 Normalized electric field

(left) and normalized concentration

Figure 2.7 Schematic overview of the distribution of two types of CIMs (or p...

Figure 2.8 Transient current measurement setup.

Figure 2.9 The schematic of the transient current measurements. (a) Measurin...

Figure 2.10 Current measurements and numerical fit for the five devices in T...

Figure 2.11 Measured (colored dots) and simulated (black lines) transient re...

Figure 2.12 Characterization of five surfactants in dodecane by current meas...

Figure 2.13 Electrical double layer in non‐polar suspension [27] American Ch...

Figure 2.14 Three charging mechanisms reported by Schreuer et al. (a) field ...

Figure 2.15 Schematic overview of the

optical trapping electrophoresis

(

OTE

)...

Figure 2.16 Histogram of 75 000 charge measurements in units of the elementa...

Figure 2.17 (a) Measurement of the z‐position of a single optically trapped ...

Figure 2.18 Optical response of EPDs with different solvent viscosity [1] / ...

Figure 2.19 Activation of particles with various temperatures in EPD [1] / J...

Figure 2.20 Forces encountered by electronic ink particles.

Figure 2.21 Depletion flocculation for bistability. (a) with free polymers a...

Figure 2.22 (a) Cross section of IMCP EPD. (b) Schematic diagram of field‐re...

Chapter 3

Figure 3.1 Multi‐gray scale of E‐paper driven by (a) PWM and (b) PAM.

Figure 3.2 (a) Bipolar and (b) unipolar driving waveforms.

Figure 3.3 Typical EPD waveform containing three stages: erasing, activating...

Figure 3.4 Demonstration of historical dependence of EPDs.

Figure 3.5 Illustration of the local DC balance method.

Figure 3.6 The driving method of Global DC balance: (a) general concept; (b)...

Figure 3.7 Bounce back effect and invisible shaking.

Figure 3.8 A complete EPD driving system.

Figure 3.9 (a) Manual waveform adjustment for an EPD with 16 grayscales and ...

Figure 3.10 Architecture of 3‐color EPDs. Black dots with negative signs: bl...

Figure 3.11 Driving waveforms for 3‐color EPDs with four stages.

Figure 3.12 Red color's chromaticity coordinate versus the voltage (left) an...

Figure 3.13 Three‐color EPD. From left to right: black‐white, red‐white, and...

Figure 3.14 Human eye's MTF, given by the equations in the right, where u

1

a...

Figure 3.15 A halftone image formed by binary pixels and its perceptual imag...

Figure 3.16 Digital halftoning methods: (a) the 4‐by‐4 Bayer matrix, a pixel...

Figure 3.17 Photographs on a 4‐bit EPD: direct quantization from an 8‐bit in...

Figure 3.18 Using the standard Floyd–Steinberg's error‐diffusion method, a 7...

Figure 3.19 Reflectance function curve depicting how fast the white state tr...

Figure 3.20 (a) Quantization errors diffused from previously processed pixel...

Figure 3.21 (a) Flow chart of the modified DBS method simultaneously conside...

Figure 3.22 Ideal electric‐optic response of a display in the sRGB color spa...

Figure 3.23 (a) Output optical response versus input grayscale; (b) tone‐map...

Figure 3.24 Image calibration for a color filter‐based full‐color EPD. (a) P...

Figure 3.25 Photographs of two images before and after applying the saturati...

Figure 3.26

Self‐powered electronic paper

(

SPEP

) integrated with a

tri

...

Figure 3.27 Illustration of the three‐dimensional driving method With Adapte...

Chapter 4

Figure 4.1 Types of reflective displays, (a) diffuse reflective, (b) specula...

Figure 4.2 Two types of simple retroreflectors, (a) spherical glass bead typ...

Figure 4.3 Description of Snell's law from rays (a)–(d) emitting from a ligh...

Figure 4.4 Monte Carlo simulation of 10 light rays entering a hemisphere typ...

Figure 4.5 Simple diagram showing optical gain of a semi‐retroreflective dis...

Figure 4.6 CLEARink Displays' Dr. Robert Fleming, CTO, and Dr. Joel Pollack,...

Figure 4.7 General description of semi‐retroreflective display using sub‐mic...

Figure 4.8 Description of the 5–30 reflected brightness test method.

Figure 4.9 Conference room, office, or classroom use case study. Overhead se...

Figure 4.10 The goniometer measurement set up (a), the measured normalized r...

Figure 4.11 Viewing and incidence angle performance study of a CLEARink Gen ...

Figure 4.12 Display performance of a CLEARink semi‐retroreflective display v...

Figure 4.13 Measured contrast ratio of a CLEARink type display and an Apple ...

Figure 4.14 The measured reflection normalized to a Lambertian white standar...

Figure 4.15 A general depiction of how color is generated using the CLEARink...

Figure 4.16 Picture of CLEARink demo shown at the Society for Information Di...

Figure 4.17 Optical micrographs of 2017 SID demo; (a) color filter array, an...

Figure 4.18 Optical micrographs of; (a) color filter array, and (b) Gen 3 ty...

Figure 4.19 Pictures of CLEARink Display's demos exhibited at the 2019 Socie...

Figure 4.20 Red‐Green‐Blue color points measured with an Ocean Optics Spectr...

Figure 4.21 E Ink 2‐particle type display. The particles are encased in micr...

Figure 4.22 CLEARink type display device. A positive voltage is applied to t...

Figure 4.23 Example of a CLEARink waveform and resulting optical state.

Figure 4.24 1 mm × 1 mm squares demonstrating 28 gray levels on a single act...

Figure 4.25 Left images shows four rows, each with a distinct gray level bei...

Figure 4.26 An optical micrograph showing sub‐pixels driven to four differen...

Figure 4.27 Optical micrographs of a full color device at the sub‐pixel leve...

Figure 4.28 Controls scheme for CLEARink active matrix display.

Figure 4.29 Power consumption on % active switching for video mode and for e...

Figure 4.30 Front light guide showing light being directed out of the light ...

Figure 4.31 Picture of an image on a CLEARink display with a front light.

Chapter 5

Figure 5.1 Examples of ChLC displays of different categories. (a) Glass base...

Figure 5.2 Illustrations of some structures of cholesteric liquid crystals....

Figure 5.3 An optically addressed cholesteric liquid crystal display.

Figure 5.4 The change in peak wavelength of (a) a sample with nearly perfect...

Figure 5.5 Illustration showing reflection from a cell with an imperfect Pla...

Figure 5.6 Microscopic picture of a PDLC where one section is electrically s...

Figure 5.7 Change in reflectivity and peak reflected wavelength of a cholest...

Figure 5.8 SEM micrographs of PDLC after substrates are removed and ChLC rin...

Figure 5.9 Example of flexible and rugged display showcasing (a) high flexib...

Figure 5.10 120% increase in color gamut and resultant color purity after ap...

Figure 5.11 Schematic of the transitions among cholesteric textures. The int...

Figure 5.12 Representative response of a bistable cholesteric liquid crystal...

Figure 5.13 Conventional drive scheme.

Figure 5.14 The electro optic response of the same displays that is initiall...

Figure 5.15 80 ppi ¼ VGA cholesteric reflective bistable display made using ...

Figure 5.16 Rugged multiplexed encapsulated cholesteric displays made using ...

Figure 5.17 The some of the colors produced by a single pixel multilayer enc...

Figure 5.18 Not to scale cross section of a typical ChLC panel used in writi...

Figure 5.19 General representation of the polymerization induced phase separ...

Figure 5.20 SEM pictures of three different types of writing tablet morpholo...

Figure 5.21 SEM micrograph of a cross section of a writing tablet. The scale...

Figure 5.22 Zoom in to the border between Planar textures created by flow an...

Figure 5.23 Writing and erasing cycle of a writing tablet.

Figure 5.24 Effect on written line intensity profiles of: (a) writing speed ...

Figure 5.25 Different writing tablet morphologies exhibiting different respo...

Figure 5.26 (a) Linewidth and peak reflectance per unit area plot for differ...

Figure 5.27 Reflectance of the written line of different writing tablets wit...

Figure 5.28 Schematic for measuring the electro‐optic response of a choleste...

Figure 5.29 A typical electro‐optic response curve of a bistable cholesteric...

Figure 5.30 Example of exact erase. (a) Original writing (b) Exact Erase (c)...

Figure 5.31 Cross section of a writing tablet showing nominal cell gap, d1, ...

Figure 5.32 Idealized illustration of the EO curves for the regions with nom...

Figure 5.33 Histograms of humans writing on a writing tablets (a) writing sp...

Figure 5.34 (a) Writing pattern designed for reliability testing. (b) Instan...

Figure 5.35 Wear points of writing tablets written on repeatedly with by a r...

Figure 5.36 The change in contrast ratio of writing tablets made with absorb...

Figure 5.37 Color contrast of semitransparent writing tablets as quantified ...

Figure 5.38 Semi‐transparent writing tablet used in tracing mode (left) and ...

Figure 5.39 A semi‐transparent display coupled to an eReader display.

Figure 5.40 A writing tablets revealing multiple colors. The one on the left...

Figure 5.41 A cross section of the writing tablet revealing multiple colors....

Figure 5.42 55‐in. diagonal bistable writing tablet.

Figure 5.43 Capturing the written image using dot/dash encoded print behind ...

Chapter 6

Figure 6.1 Zenithal bistable states of the nematic director close to a deep ...

Figure 6.2 Example modes of a ZBD, based on choice of the opposing monostabl...

Figure 6.3 Example of the electrical latching between the D‐ and C‐states ob...

Figure 6.4 The electro‐optic response of typical Zenithal Bistable Devices. ...

Figure 6.5 Three‐dimensional structure of the ZBD grating. (a) Polarizing op...

Figure 6.6 Multistep replication process that allows high speed mass product...

Figure 6.7 Hard contact step and repeat exposures used to make the ZBD maste...

Figure 6.8 (a) Scanning Electron Micrograph of a typical groove profile of Z...

Figure 6.9 Schematic of process for transferring grating profile from nickel...

Figure 6.10 A simplified process flow for the manufacture of ZBD LCDs.

Figure 6.11 Definition of adhesion energies and subsequent layer retention a...

Figure 6.12 Examples of typical active components to promote adhesion of ZBD...

Figure 6.13 Schematic of production embossing process.

Figure 6.14 SEM section showing an optimum ZBD grating. The pitch of the gra...

Figure 6.15 (a)

Progressive line blanking

(

PLB

) addressing scheme for ZBD. (...

Figure 6.16 Two possible arrangements (e‐mode or o‐mode) of the front polari...

Figure 6.17 Modeled reflectivity of the bright (TN) state and the dark (HAN)...

Figure 6.18 Modeled and measured color coordinates of the ZBD LCD white stat...

Figure 6.19 Scattering from conventional (Gaussian) diffusers and structural...

Figure 6.20 Reflectivity of two ZBD LCDs employing conventional (left) and s...

Figure 6.21 Configuration of ZBD/OLED dual layer display and resulting refle...

Figure 6.22 Dual mode display in which a ZBD LCD is fabricated on an active‐...

Chapter 7

Figure 7.1 Design concept of

memory in pixel

(MIP) reflective LCD.

Figure 7.2 System block diagram and equivalent pixel circuit of a convention...

Figure 7.3 Equivalent MIP circuit with applied signals to drive a liquid cry...

Figure 7.4 Display image data transfer of a conventional active‐matrix LCD....

Figure 7.5 Display image data transfer of an MIP LCD.

Figure 7.6 (a) “3 divided patterns,” and (b) 2‐bit gradation expressed by “3...

Figure 7.7 Structure of a color reflective LCD with scattering reflective el...

Figure 7.8 Structure of a color reflective LCD with a light control film.

Figure 7.9 Reflective optical switching mechanism of ECB and VA mode LCDs.

Figure 7.10 Cross sectional diagram of a conventional transflective LCD pixe...

Figure 7.11 Cross sectional diagram of a MIP reflective LCD pixel.

Figure 7.12 Typical V‐R and V‐T curves of a single gap ECB normally‐black LC...

Figure 7.13 2‐bit gradation expressed by the fringe of the pixel electrode....

Figure 7.14 VCOM driving method to avoid flickering issue with backlight.

Figure 7.15 Comparison of how color depth reduction and error diffusion proc...

Figure 7.16 Picture and key specifications of a 1.2 in. 240 × 240 2bit/color...

Figure 7.17 Picture and key specifications of a 1.04″ 208 × 208 1‐bit/color ...

Figure 7.18 System block diagram of a 1.04″ 208 × 208 1‐bit/color MIP LCD.

Figure 7.19 VCOM/FRP and XFRP signals generated in LCD from COMIN input sign...

Figure 7.20 VCOM control signal generated by real time clock circuits to dri...

Figure 7.21 Power consumption comparison of transmissive LCD, and reflective...

Chapter 8

Figure 8.1 Qualitative interpretation of the photo‐induced order in photoche...

Figure 8.2 Dependence of the photo‐induced phase retardation

δ

on expos...

Figure 8.3 Azimuthal LC anchoring energy of the photoaligned SD1 layer as a ...

Figure 8.4 Operational principle of an ORW LCD: Azo‐dye alignment film rotat...

Figure 8.5 Fabrication process flows of (a) photoaligned SD1 substrate and (...

Figure 8.6 Assembly of ORW LCD panel and rewriting process.

Figure 8.7 (a) The schematic ORW LCD panel in which two alignment domains wi...

Figure 8.8 Angular dependence of the ORW LCD: (a) reflectance coefficient, (...

Figure 8.9 The experimental arrangement for the optical rewriting time measu...

Figure 8.10 (a) Plot of azimuthal anchoring energy in 10 μm cell gap ORW LCD...

Figure 8.11 Schematic diagram of twisted nematic LC cell (a) without and (b)...

Figure 8.12 Average effective rotational viscosity

(right axis) and rewrit...

Figure 8.13 (a) Structure of LC‐based light printer, which consists of 1) po...

Figure 8.14 Grayscale images on ORW LCD by LC‐based light printer: (a) gray ...

Figure 8.15 Grayscale images on ORW LCD by pixel division mechanism. (a) 4‐i...

Figure 8.16 Schematic diagram to illustrate the fabrication of three alignme...

Figure 8.17 (a) The output polarization plane azimuths of the light from the...

Figure 8.18 (a) The structure of the autostereoscopic 3D ORW LCD panel. (b) ...

Figure 8.19 (a) Schematics of ORW LCD for realizing red with patterned refle...

Figure 8.20 (a) Schematic process of spacer printing. (b) Top view of spacer...

Figure 8.21 (a) Schematic structure of the dye‐doped ORW LCD. (b) Plots of d...

Chapter 9

Figure 9.1 Relation between lightness, the perceived white level, and the pe...

Figure 9.2 Munsel representation of the color space, with pure white at the ...

Figure 9.3 From left to right respectively: creation of white (most left fig...

Figure 9.4 A droplet of water on a hydrophobic surface does not wet the surf...

Figure 9.5 (left) A layer of oil on a hydrophobic surface wets the surface u...

Figure 9.6 Electrowetting display, partly in closed state (left) and partly ...

Figure 9.7 A video still, Ref. [10], demonstrating high contrast in a reflec...

Figure 9.8 Three optically coupled electrowetting display bipanes can create...

Figure 9.9 Etulipa's reflective

electronic changeable copy board

(

ECCB

) with...

Figure 9.10 Illustration of equation of Young‐Dupré: the contact angle, or w...

Figure 9.11 Calculated relative reflectivity as function of cell area under ...

Figure 9.12 Illustrations of electrowetting display architectures to create ...

Figure 9.13 Overview of reported switching from references [2, 3, 5, 12,30–3...

Figure 9.14 An example of Etulipa 10

″

CMY electrowetting color tile (l...

Figure 9.15 High‐speed full color TFT electrowetting displays reported by So...

Figure 9.16 Examples of reported ITRI transparent 6

″

(left) and 14

″

...

Figure 9.17 Examples of Liquavista reflective electrowetting color displays....

Figure 9.18 Examples of reported color gamuts from (left) Liquavista [30], (...

Figure 9.19 Example of light transmission as function of duty cycle of drivi...

Figure 9.20 Illustration of electrowetting display architectures with above,...

Figure 9.21 Basic steps of the process flow of Etulipa production line.

Figure 9.22 Bus stop in New York City with electrowetting displays indicatin...

Chapter 10

Figure 10.1 Electrochromic Applications: (a) smart windows, Source: Sageglas...

Figure 10.2 The typical structure and coloration mechanism of EC cell.

Figure 10.3 Schematic representation of coloration mechanism of WO

3

based EC...

Figure 10.4 Schematic representation of viologen derivative‐modified TiO

2

na...

Figure 10.5 Digital camera images of colored state of terephthalate derivati...

Figure 10.6 Digital camera image of the three‐layered ECD with an 8 × 8 pixe...

Figure 10.7 Color representation of stacked phthalate‐based EC cell, two of ...

Figure 10.8 Demonstrated image of active matrix 3.5” QVGA LTPS‐TFT multi‐lay...

Figure 10.9 Schematic diagrams and mechanism of the “Urea‐N+Rh‐M” electrochr...

Figure 10.10 Fabrication process for lateral paper‐ECDs showing inkjet‐print...

Figure 10.11 (a) Structure of Ag electrodeposition‐based EC display by SONY,...

Figure 10.12 Digital camera images of Ag electrodeposition‐based EC cell. Up...

Figure 10.13 Digital camera images of Ag electrodeposition‐based EC cell exh...

Chapter 11

Figure 11.1 Diagram illustrating the switching signals (S

1

and S

2

) input to,...

Figure 11.2 Illustrated reflection process and corresponding phasor diagram ...

Figure 11.3 The Mirror‐spacer‐PCM‐capping layer stack design (a), resulting ...

Figure 11.4 Illustrated device schematic (a) and resultant visual appearance...

Figure 11.5 FEA visualizations of the improved buried microheater pixel, sho...

Figure 11.6 Reflection spectra of PCM interference stacks designed to switch...

Figure 11.7 (a) Cross‐section schematic of an RGB sub‐pixelated PCM stack, w...

Figure 11.8 CIE 1931 chromaticity diagram showing the color gamut produced b...

Figure 11.9 A high‐throughput, combinatorial sample of the GST ternary diagr...

Figure 11.10 Microscope photographs of a 2 × 2 pixel region of the PARC LTPS...

Figure 11.11 Microscope photographs of a 3 × 4 pixel region of the Tianma LT...

Figure 11.12 Microscope photographs of an array of 24 × 24 SRD pixels being ...

Figure 11.13 (a) Schematic illustration of the prototype array showing the l...

Figure 11.14 Examples of LTPS SRD backplanes for 1.3

″

, 480 x 480 pixel...

Figure 11.15 Photographs of the 480 × 480 pixel SRD display prototype, showi...

Figure 11.16 Structural schematic (a), and resultant projected image appeara...

Figure 11.17 (a) Schematic design of silicon backplane transistor switched P...

Chapter 12

Figure 12.1 Fundamentals of reflection for incident and reflected light in t...

Figure 12.2 Examples for bright (left) and dark (right) state of diffuse fla...

Figure 12.3 Measurement set‐up for angle resolved reflectance measurement (l...

Figure 12.4 Reflective displays measurement set‐ups: Sampling sphere (left) ...

Figure 12.5 Effect on the measured reflections of typical surface characteri...

Figure 12.6 Measurement set‐up (left) with luminance (or color) imager and s...

Figure 12.7 Visual comparison of an electrophoretic display and a reflective...

Figure 12.8 Example of a character contrast ratio measurements using an imag...

Figure 12.9 Comparison of the contrast ratio for diffuse illumination of a c...

Figure 12.10 Color performance of reflective displays in various color space...

Figure 12.11 Standard updating of a professional electrophoretic display (le...

Figure 12.12 Lifetime testing: Left: Typical reduction of reflectance (e.g. ...

Guide

Cover Page

Series Page

Title Page

Copyright Page

List of Contributors

Series Editor's Foreword

Editor's Preface

Table of Contents

Begin Reading

Index

WILEY END USER LICENSE AGREEMENT

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Wiley‐ SID Series in Display Technology

Series Editor: Dr. Ian Sage

Advisory Board: Paul Drzaic, Ioannis (John) Kymissis, Ray Ma, Ian Underwood, Michael Wittek, Qun (Frank) Yan

E‐Paper DisplaysBo‐Ru Yang (Ed.)

Liquid Crystal Displays ‐ Addressing Schemes and Electro‐Optical Effects, Third EditionErnst Lueder, Peter Knoll, and Seung Hee Lee

Flexible Flat Panel Displays, Second EditionDarran R. Cairns, Dirk J. Broer, and Gregory P. Crawford

Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and MemoryHideo Hosono, Hideya Kumomi

Introduction to Flat Panel Displays, Second EditionJiun‐Haw Lee, I‐Chun Cheng, Hong Hua, and Shin‐Tson Wu

Flat Panel Display ManufacturingJun Souk, Shinji Morozumi, Fang‐Chen Luo, and Ion Bita

Physics and Technology of Crystalline Oxide Semiconductor CAAC‐IGZO: Application to DisplaysShunpei Yamazaki, Tetsuo Tsutsui

OLED Displays: Fundamentals and Applications, Second EditionTakatoshi Tsujimura

Physics and Technology of Crystalline Oxide Semiconductor CAAC‐IGZO: FundamentalsNoboru Kimizuka, Shunpei Yamazaki

Physics and Technology of Crystalline Oxide Semiconductor CAAC‐IGZO: Application to LSIShunpei Yamazaki, Masahiro Fujita

Interactive Displays: Natural Human‐Interface TechniquesAchintya K. Bhowmik

Addressing Techniques of Liquid Crystal DisplaysTemkar N. Ruckmongathan

Modeling and Optimization of LCD Optical PerformanceDmitry A. Yakovlev, Vladimir G. Chigrinov, and Hoi‐Sing Kwok

Fundamentals of Liquid Crystal Devices, Second EditionDeng‐Ke Yang and Shin‐Tson Wu

3D DisplaysErnst Lueder

Illumination, Color and Imaging: Evaluation and Optimization of Visual DisplaysP. Bodrogi, T. Q. Khan

Liquid Crystal Displays: Fundamental Physics and TechnologyRobert H. Chen

Transflective Liquid Crystal DisplaysZhibing Ge and Shin‐Tson Wu

LCD BacklightsShunsuke Kobayashi, Shigeo Mikoshiba, and Sungkyoo Lim (Eds.)

Mobile Displays: Technology and ApplicationsAchintya K. Bhowmik, Zili Li, and Philip Bos (Eds.)

Photoalignment of Liquid Crystalline Materials: Physics and ApplicationsVladimir G. Chigrinov, Vladimir M. Kozenkov, and Hoi‐Sing Kwok

Projection Displays, Second EditionMathew S. Brennesholtz and Edward H. Stupp

Introduction to MicrodisplaysDavid Armitage, Ian Underwood, and Shin‐Tson Wu

Polarization Engineering for LCD ProjectionMichael G. Robinson, Jianmin Chen, and Gary D. Sharp

Digital Image Display: Algorithms and ImplementationGheorghe Berbecel

Color Engineering: Achieving Device Independent ColorPhil Green and Lindsay MacDonald (Eds.)

Display Interfaces: Fundamentals and StandardsRobert L. Myers

Reflective Liquid Crystal DisplaysShin‐Tson Wu and Deng‐Ke Yang

Display Systems: Design and ApplicationsLindsay W. MacDonald and Anthony C. Lowe (Eds.)

E‐Paper Displays

Edited by

Bo‐Ru Yang

State Key Lab of Opto‐Electronic Materials & Technologies,

Guangdong Province Key Lab of Display Materials and Technologies,

School of Electronics and Information Technology,

Sun Yat‐Sen University, Guangzhou, China

This edition first published 2022© 2022 John Wiley & Sons Ltd

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

The right of Bo‐Ru Yang to be identified as the author of the editorial material in this work has been asserted in accordance with law.

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Limit of Liability/Disclaimer of WarrantyIn view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

Library of Congress Cataloging‐in‐Publication DataNames: Yang, Bo‐Ru, 1979– editor.Title: E‐paper displays / edited by Bo‐Ru Yang, State Key Lab of Opto‐Electronic Materials & Technologies, Guangdong Province Key Lab of Display Materials and Technologies, School of Electronics and Information Technology, Sun Yat‐Sen University, Guangzhou, China.Description: First edition. | Hoboken, NJ : John Wiley & Sons, 2022. | Series: Wiley SID series in display technology | Includes bibliographical references and index.Identifiers: LCCN 2021061537 (print) | LCCN 2021061538 (ebook) | ISBN 9781119745587 (hardback) | ISBN 9781119745594 (adobe pdf) | ISBN 9781119745600 (epub)Subjects: LCSH: Electronic paper.Classification: LCC TK7882.E44 E24 2022 (print) | LCC TK7882.E44 (ebook) | DDC 004.5/6–dc23/eng/20220207LC record available at https://lccn.loc.gov/2021061537LC ebook record available at https://lccn.loc.gov/2021061538

Cover Design: WileyCover Image: © Marko Aliaksandr/Shutterstock.com

List of Contributors

Karlheinz BlankenbachPforzheim UniversityDisplay LabPforzheim, Germany

Clinton BraganzaPortage BlvdKent, USA

Ben BroughtonOxford, UK

Guy P. Bryan‐BrownDirector of TechnologyNew Vision Display, Inc.Malvern, UK

Anne ChiangChiang Consulting,Cupertino, USA

Vladimir ChigrinovState Key Laboratory of Advanced Displays and Optoelectronics TechnologiesHong Kong University of Science and TechnologyHong Kong S. A. R., China

Paul S. DrzaicApple, Inc.CA, USA

Mauricio EcheverriKent Displays Inc.Portage BlvdKent, USA

Robert J. FlemingCTO CLEARink DisplaysSan Jose, CA, USA

Yoko FukunagaJDI Higashiura PlantAichi‐ken, Japan

Peiman HosseiniBodle TechnologiesOxford, UK

J. Cliff JonesSchool of Physics and AstronomyUniversity of LeedsCheshire, UK

Norihisa KobayashiGraduate School of Engineering Chiba UniversityJapan

Hoi‐Sing KwokState Key Laboratory of Advanced Displays and Optoelectronics TechnologiesHong Kong University of Science and TechnologyHong Kong S. A. R., China

Kristiaan NeytsProfessor and Head of Liquid Crystals and Photonics Group, ELIS Department, Ghent University,Belgium

Doeke J. OostraEtulipa, a Miortech companyThe Netherlands

Zong QinSchool of Electronics and Information TechnologySun Yat‐Sen UniversityGuangzhou, China

Abhishek SrivastavaState Key Laboratory of Advanced Displays and Optoelectronics TechnologiesHong Kong University of Science and TechnologyHong Kong S. A. R., China

Wanlong ZhangState Key Laboratory of Advanced Displays and Optoelectronics TechnologiesHong Kong University of Science and TechnologyHong Kong S. A. R., China

Series Editor's Foreword

Paper is the medium which has dominated the presentation of both the written word and graphics for two thousand years. The happy partnership of paper with printing was first developed in China before being adopted in the Western world in the Middle Ages and it initiated the first information explosion. Perhaps no other invention has had such a fundamental influence on the development of human society and way of life. Printed books and periodicals are attractive, affordable, and so familiar that we can truly pay attention to the contents alone, and overlook the qualities of the physical page. With this cultural and technical background, it is not surprising that printed paper is widely regarded as a reference point for electronic display technologies and since the introduction of the first flat panel displays, achieving paper‐like performance has been a common dream and aspiration of engineers. At the same time, users have bemoaned the shortcomings of electronic displays and the poor reading experience offered by early generations of displays.

In the present volume, Professor Bo‐Ru Yang has brought together an outstanding selection of authors to examine the technologies which aspire to mimic the properties of printed paper. It is fair to say that no electronic display can yet rival all the desirable characteristics of paper, and a basic question which arose at an early point in the planning of the volume, was which technologies or aspects of performance should be included. Professor Yang and his team have taken an inclusive approach. In this book the reader will find accounts of displays which offer different combinations of desirable properties including paper‐like appearance under ambient light, long‐term image storage with zero or ultra‐low power consumption, light weight and flexibility, and the ability to accept user input with the ease of pencil on paper. It follows that a wide range of display technologies are included, with an emphasis on device modes which offer ambient light viewability. Several liquid crystal modes, both with and without polarizer, can provide image storage either intrinsic to the display or in ultra‐low power drive electronics. Meanwhile, displays based on electrophoretic or electrowetting effects can offer outstanding optical appearance under a wide range of ambient lighting. Innovative and emerging displays are also considered, with a chapter on phase‐change displays offering an early view of the potential of this new development. Of course, physics of operation, the fabrication, engineering and especially the addressing characteristics of each display mode can be very different, and these issues are comprehensively covered, with special attention to those aspects of the devices which are less well covered in earlier sources. Other, user‐related properties of the devices—the difficulty of providing high quality reflective colour, and the relation of the display performance to human perception of image quality and metrology are carefully considered.

In the 21st century, our priorities regarding electronic displays have changed. Excellent optical performance is today taken for granted while convenience, light weight and long battery life have increasingly driven user approval. Especially, the environmental impact of every activity we undertake, now demands scrutiny. Printed media are major consumers of environmental resources including energy, timber, carbon emissions and chemical residues while electronic devices have the potential to reduce or eliminate these problems—but only if they are responsibly manufactured and accepted by users sufficiently to displace print over a long period of use and wide range of use cases. Paper‐like display quality can really make a difference to the world. The present book provides a comprehensive and authoritative overview of the field, authored by leading experts on each aspect of the subject, and I believe it will offer a most valuable source of reference to display professionals and advanced students, for many years to come.

Ian SageMalvern

Editor's Preface

With the advent of the Internet of Things (IoT), devices and objects around us are being equipped with built‐in sensors and electronics which allow them to analyse and share information in real time, in the manner we associate with intelligent organisms. It follows that vast numbers of devices will be fitted with displays to present the information. The needs of such autonomous miniature devices mean that displays with low power consumption, excellent sunlight readability, and geometrical conformability which are compatible with low‐cost fabrication methods, are becoming critical components in future IoT environments. E‐paper displays have the inherent advantages of reflective operation, zero‐power bistability, and in many cases they can be fabricated by printing‐based processes. Therefore, they are regarded as among the most promising display technologies for these applications. Furthermore, many recent applications, such as fixed and mobile signage for transport, advertising billboards, architectural coatings, wrapped vehicles, e‐readers, retail labelling, dynamic artworks, and many others, have started to exploit the unique advantages of E‐paper.

Unlike LCD and OLED technologies, E‐paper display technologies have not up until now been collectively and comprehensively reviewed, and there has been no published source which can provide scientists, engineers, and users with enough broad, insightful, and up‐to‐date knowledge to support research and development or product integration in this field. To fill this need, the present book represents the achievement of a three‐year collaboration with prestigious scholars, experts, and entrepreneurs in E‐paper display fields.

In this book, we have tried to cover as extensive a range of E‐paper technologies as possible. Started with the development history of electrophoretic displays, followed by the fundamental mechanisms, physical models, driving waveforms, image processing, and advanced structures of electrophoretic displays, we describe the technological details and review the development progress to show how electrophoretic E‐papers become commercially successful. In addition, bistable and reflective LCDs for E‐paper applications, including Cholesteric LC, Zenithally Bistable Display (ZBD), Memory in Pixel (MIP), and Optically Rewritable (ORW) LCDs are also introduced. After that, the emergent technologies such as electro‐wetting, electro‐chromic, and phase change materials for E‐paper display applications are reported and summarized. In the last part, the special needs for metrology of E‐paper displays are explained. This book covers the broad spectrum of state‐of‐the‐art reflective and bistable E‐paper technologies, and we believe it will provide an invaluable handbook and reference for researchers, advanced students, and professionals in this field.

1The Rise, and Fall, and Rise of Electronic Paper

Paul S. Drzaic1, Bo‐Ru Yang2, and Anne Chiang3

1 Apple, Inc

2 Professor, School of Electronics and Information Technology, State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, Sun Yat‐Sen University, Guangzhou, China

3 Principal, Chiang Consulting, Cupertino, CA

1.1 Introduction

For over a thousand years, before the world of electronics, paper was the dominant medium for people to share written and later printed information. People become familiar with paper at an early age, and there is an enormous worldwide infrastructure for the production and distribution of printed material. Despite this huge built‐in advantage, paper and print now fall short in providing for many demands of modern life. The past few decades have seen the emergence of electronic networks that transmit vast amounts of information, on‐demand, for use in various ways. Electronic displays are a necessary part of this infrastructure, converting bits to photons and serving as the final stage of transmitting information to people.

Over the years, several electronic display technologies have waxed and waned; cathode ray tubes, plasma displays, and super twisted nematic (STN) displays come to mind. A few technologies are dominant; backlit active‐matrix liquid crystal displays, active matrix organic light‐emitting diode (OLED) displays, and inexpensive, passively addressed liquid crystal displays. Along the way, tens of different types of display technologies have been invented and explored, but ultimately have failed to catch on. A few displays have found a home in niche products and promise greater future application. Reflective displays, particularly electronic paper, are examples that have managed to find a place in the display ecosystem, with unique applications best served by these technologies.

This book aims to update on some of the most exciting new areas in electronic paper technology. This introductory chapter focuses primarily on electrophoretic displays (EPDs) and how they became synonymous with electronic paper. The story starts in the early 1970’s, with the proposal and first demonstration of the electrophoretic movement of charged particles to make an optical effect. After intense effort, the technology was mostly abandoned, only to be resurrected by a start‐up company, E Ink. Several key, and rather improbable inventions had to be made to develop a technology competitive to the dominant liquid crystal technology. Finally, the right application and ecosystem, the Amazon Kindle electronic book, was necessary to cement commercial success.

The field of reflective displays is very rich. The many other chapters in this book and recent reviews [1, 2] provide a wealth of resources for understanding the many technologies that have been developed in the quest to achieve a paper‐like display. In this chapter, we will examine the following:

A description of print‐on‐paper and how the optics of real paper compare with potential electronic paper competitors.

A hierarchical summary of the different technical approaches for reflective displays.

A detailed look at the historical development of EPDs, starting with the invention of the technology and ending with the introduction of the Amazon Kindle. Looking at the various developments in the context of its times, the EPD story offers some lessons in what it takes for a technology to transition from the laboratory to commercial success.

1.2 Why Electronic Paper?

Electronic paper has undoubtedly caught the imagination of the world. A Google search for electronic paper in September 2021 returns over 12 billion hits. This interest reflects people's love affair with paper as a medium for transmitting information. Yet, it is easy to recognise that printing ink onto dead trees is not easily compatible with today's networked world. What are the attributes of print‐on‐paper that make it so important?

Paper is a reflective medium that automatically adapts to changing lighting

Unlike most emissive displays, paper can be easily read in bright sunlight.

The appearance of paper is relatively constant over different viewing angles, without significant shifts in luminance or color.

Paper can be lightweight and flexible. The user can easily annotate it. with a pen or pencil.

Paper is inexpensive

Paper can be archived.

Nevertheless, physical paper cannot be instantly updated with information from electronic networks or easily serve as an interface with electronic devices. Today's backlit LCDs and OLED displays are ubiquitous as a means of transmitting information, but with the limitations that emissive displays possess, including eyestrain and low visibility in sunlight. Electronic paper can combine the power of electronic devices and networks with all the attributes of paper.

So what strategies can be taken to enable electronic paper? It is instructive to understand the composition and design of print‐on‐paper and see how many of these properties can be converted to something under electronic control to compete with printed media.

1.3 Brightness, Color, and Resolution

Conventional, non‐electronic paper consists of a mat of tightly pressed fibers, most commonly derived from parchment or wood pulp. The combination of fibers and embedded air pockets scatter light and provide the reflective characteristics of paper. Historically, additives to the paper pulp during fabrication have also provided glossiness, color, aid in manufacture, or other desirable characteristics (Figure 1.1).

Compared to a white optical standard, the perceived reflectivity of paper often ranges from 50–80%, but can be even higher. The whiteness or brightness of paper depends on several factors, including the density of fibers, the paper thickness, the presence of additives such as titanium dioxide, clays, or fluorescent agents, and whether the viewing surface is made glossy through calendaring and coatings. The color of light reflected from white paper may differ somewhat from a perfect reflector due to the fluorescing whiteners' presence, or some underlying color absorbance from the paper. The human eye readily accommodates for these changes, though, so the perception of consistent color and lightness of a page relative to its surroundings is easily achieved (Figure 1.2).

Print‐on‐paper consists of drops of colored ink impregnated into the paper fiber. It is straightforward to devise dyes and pigments that absorb red, green, or blue. To generate the color characteristics of print, the CMYK subtractive color system can be used (Figure 1.3). The colors in print are usually comprised of cyan (absorb red), magenta (absorb green), and yellow (absorb blue) (Figure 1.4). Black pigment (the K in CMYK) is also commonly used, as it is challenging to achieve a neutral black color by mixing cyan, magenta, and yellow.

Figure 1.1 Arches 100% cotton rag paper. Scanning electron microscope image @100×.

Source:http://paperproject.org,[3] Used with permission of CJ Kazilek.

Figure 1.2 CIELAB color system [4] John Wiley & Sons.

Figure 1.3 Color separation of an image into its CMYK components, and the final printed image. The absence of a color is white.

Figure 1.4 Subtractive color mixing. Cyan and magenta overlap to make blue, cyan, and yellow overlap to make green, yellow, and magenta overlap to make red, and all three mixed together provide black.

To achieve a wider color gamut, inkjet printing may use six or more colors to print. Additionally, spot color printing can deposit specialty inks (such as fluorescent pigments) that currently have no analog in emissive displays.

Depending on the industry, a variety of different color spaces and metrics have been developed for printed and reflected color. For example, the CIE 1976 (L*a*b*) color space is widely used to measure reflective colors and print. The human perception of lightness is measured by L*, which roughly scales as the cube root as the reflected luminance level.

For color reproduction in the print industries, the SNAP standard (Specifications for Newspaper Advertising Production) and SWOP standard (Specifications for Web Offset Publications) are widely used [1]. These print standards are rarely applied to electronic displays, though the advent of colored reflective displays approaching print‐like appearance could change this situation.

Grayscale in printing is achieved using halftones (Figure 1.5). Each dot on a printed page defines an area where smaller halftone dots are printed. The more halftone dots, the deeper the color or darker the black while white is the absence of halftone dots. The smaller the dots, the higher the resolution. Print is often defined in lines per inch.″ Some examples of everyday printed objects include:

Newspaper (monochrome) – 65–100 LPI

Books and magazines (color) – 120–150 LPI

Art books (color) – 175–250 LPI

Photorealistic inkjet printer (color) – 250–300 LPI

Likewise, the resolution‐defined dot for color printing consists of multiple prints of smaller dots. The combination of different colors and black and the underlying brightness and color of the paper provide the specific color and lightness of that dot.

With inkjet printing, commercial printers can also control the size of the drop, such as achieving four different sizes (Figure 1.6). The number of achievable gray levels is a combination of the number and size of the printed dots within the equivalent printed pixel [6, 7].

Figure 1.5 Examples of halftone dots of variable size, enabling grayscale [5] / Slippens / Public Domain.

1.4 Reflectivity and Viewing Angle

An important aspect of paper is that the image printed on the page appears to have constant lightness and color irrespective of the viewing angle and lighting conditions. If the paper is glossy, there may be some glare from the surface, but that reflectance rarely interferes with the user interacting with the page. The near‐constant appearance of the printed page is representative of Lambertian reflectance. The underlying paper scatters light, impinging onto the surface from many angles and then reflected uniformly into all spatial angles. Whether the page is illuminated by a collimated source such as a light bulb in a dark room or a more uniform source such as a cloudy sky, the distribution of reflected light from the paper does not change that much. Likewise, the amount of light scattered into the viewer's eyes for a given spot on the paper is also constant with viewing angle [8].

Figure 1.6 Printed color generation through printed CMYK halftones [5] / Slippens / Public Domain.

1.5 Translating Print‐on‐Paper into Electronic Paper

With this background, we can examine why designing an electronic display with the appearance of high‐quality print‐on‐paper is challenging.

1.5.1 Brightness

Electronic displays require a transparent front sheet as the top surface of the display. This top surface (usually plastic or glass) provides a flat dielectric interface with air and degrades the optics of electronic paper in several ways.

A significant fraction (4–10%) of light is directly reflected off the display without interacting with the display medium. This reflected light is seen as glare and degrades the contrast of the underlying display to the viewer. This reflected light also reduces the available light scattered back to the viewer. Anti‐reflective coatings can reduce this reflected light, but with added cost and sometimes compromises in durability.

Even if there is perfect diffuse reflectance, a significant fraction of that light will be trapped by internal reflectance at the display‐air interface

[9]

. For an EPD where the dielectric phase is somewhere in the range of 1.38–1.45, the maximum amount of light that can be outcoupled from a white scattering layer, with no other absorptive losses, is predicted to be no more than 65%. These incoming and outcoupling losses limit the potential brightness of an embedded Lambertian scatterer within an electronic paper display.

There are strategies to improve the luminance of scattering‐based displays by incorporating focusing optical elements with the display itself. Fleming et al. have demonstrated a display that uses a retroreflective prismatic element within the display and a black electrophoretic colloid to control the reflection from this element [10, 11]. While the reflectivity of the micro prism layer will depend on the geometric details of the source illumination, in many lighting conditions, the micro prism itself provides optical gain, and the display can appear “whiter than white.”[12]. One display by Fleming et al., when illuminated by moderately collimated light, is reported to have an apparent brightness over 80%, with a 20: 1 contrast ratio. Fleming will describe more information on these types of displays in Chapter 4 of this volume. Further information about the metrology of measuring reflective display and e‐paper will be described in Chapter 12.

1.5.2 Grayscale – Analog vs. Digital

High‐quality images require grayscale, with 256 levels for each color channel being standard in emissive displays. Today, it is impractical to rely only on halftoning approaches to generate high‐quality grayscale, which would require further subdividing each CMYK subpixel into an additional 4, 8, or 16 subpixels to maintain the native resolution. Instead, grayscale is generated using analog techniques, in which any pixel can be programmed to show intermediate reflective states between black and white. While many reflective display technologies, including EPDs, can show continuous grayscale within any pixel, maintaining accurate and uniform grayscale across all pixels is challenging. Analog gray levels are typically restricted to a number that enables uniform luminance without mura artifacts. For example, today's commercial e‐paper displays from E ink are limited to 16 analog levels of gray to achieve good uniformity. However the underlying technology is capable of more gray levels [13].

Sophisticated dithering algorithms have been developed to minimize perceptual errors, some of which have been examined for use in electronic displays [14]. Dithering is analogous in many ways to halftones in print. In an electronic display, dithering sacrifices resolution to give the appearance of intermediate gray levels or to suppress non‐uniformities between pixels.

1.5.3 An Overview of Approaches to Color Electronic Paper

Many inventive approaches have been proposed to achieve electronic color paper. Some designs take advantage of an electro‐optical medium that can change color at the subpixel level intrinsically. Other approaches use color filters with an otherwise colorless black and white effect. Accurate and uniform grayscale is essential for high‐quality images and is often very difficult to achieve.

Figure 1.7 illustrates an EPD in combination with color filters [15]. The function of a color filter is to absorb portions of the visible spectrum so that reflected light is colored. RGB or CMY primary colors can blend the primaries to provide an extended color range. Grayscale is adjusted by controlling the state of the reflective medium, similar to gray in black and white displays. These displays inevitably must make trade‐offs between color saturation, color space coverage, and brightness. Reasonable brightness is usually achieved at the cost of color purity and a restriction of available colors.

Another approach is the adoption of multiple color particles as electrophoretic elements [16, 17]. Different colors can be generated in each sub‐pixel by mixing C, M, Y translucent particles with scattering white particles, as shown in Figure 1.8. This approach was firstly demonstrated by Fujifilm in 2012 [16], and later on by E ink [17], which named it Advanced Color ePaper (ACeP). The colored pigments have different electrophoretic mobilities and responsivities, which facilitate to shuffle the color particles with driving waveforms of different voltage and pulse widths. As shown in Figure 1.8, different primary colors can be presented by positioning the white particle layer above or underneath the translucent C, M, Y particles. Integrating the CMYW particles into a display unit, ACeP can present different colors and gray levels in each subpixel by controlling the driving waveforms. These driving waveforms are complex, and frame rates are currently slower than other EPD approaches. Nevertheless, the multiple color particle design dramatically increases the color saturation and reflective luminance achievable compared to the color filter approach. Figure 1.9 compares the color performance of these two types of color EPD prototypes [16, 17].

Figure 1.7 Color electrophoretic display using B/W particles and subpixel color filters [15] / John Wiely & Sons.

Figure 1.8 Eight primary colors of ACeP by different pigment arrangements [17] / John Wiely & Sons.

Many other inventive approaches have tackled the problem of making the electro‐optical medium capable of switching color. Controlled lateral migration of colored fluid and particles enables a bi‐primary color system. This design allows a single pixel to be changed through mixtures of color states [19].

A variety of other physical phenomena can be used to modulate reflected light electrically. The following table describes several varieties of reflective displays that have been developed over the past several decades. The fact that so many technologies have been pursued shows both the interest in the electronic paper and the difficulty in achieving a paper‐like display. We compare the various reflective display technologies in Table 1.1, showing the strengths and weaknesses of the different approaches. More information on these different technologies can be found in reviews [1, 2, 20] and other chapters in this book.

Figure 1.9 Prototypes of colored electrophoretic displays: the left image uses a color filter and front light, while the middle and right images use the multiple color particles design [16–18] A. [18] Ian French, et al. 2020; B.[16