Designing e-Books That Will Be Comfortable to Use

Although documents can be read on computer screens, paper is still preferred. When will there be an e-book that will be comfortable to read for hours at a time?

by Mark T. Johnson and Guofu Zhou

AN UNPARALLELED amount of electronic information is now available in the form of text and graphics, but do we want to read a book on our laptop, telephone, or PDA display for hours at a time? Of course not.

Without a backlight, the displays are just too dark and the reading experience is uncomfortable. What we need is an e-book with a display that looks and even feels like paper. Electronic-paper displays for e-books, with performance identical to that of conventional paper in terms of brightness and contrast, are the holy grail of the display industry. They would permit the immersional reading of a display for hours at a time as we do a book.

Expectations for e-Books

Makoto Omodani highlighted the features expected from electronic-paper displays in his article in Information Display, "Expecting Readability," in December 2004. The factors that make reading from paper more appealing than from a computer screen are

• Readability, due to a combination of high resolution, high reflectivity, and insensitivity to viewing angle and lighting conditions,

• Portability and comfortable hand-held reading (as opposed to reading from a fixed screen), due to a thin and light form factor as well as flexibility,

• Ultra-low power consumption, with no power required to maintain the image once written.

An ideal e-book display would combine all of these features.

LCDs for e-Books

Although the obvious technology to satisfy this demand is liquid-crystal-display (LCD) technology, which now dominates the flat-panel-display (FPD) industry, LCDs have not been widely adopted for this application.

In most LCDs, the liquid crystal modulates the polarization state of the light that passes through the display, resulting in the loss of a significant amount of light in the polarizer. For this reason, a nematic LCD cannot be as bright as paper, although impressive low-power displays have been demonstrated, such as those by Nemoptic.¹

Other types of liquid crystals can operate without polarizers, modulating light using scattering (polymer-dispersed LC) or absorbing (guest-host LC) modes of operation.^2,3 Although these LC effects are inherently brighter than those of nematic LC, turning off the power results in the loss of the image, so the low-power aspect of electronic paper is not really met.

One LC mode that combines low power comsumption and relatively high brightness is the cholesteric textured LC (CTLC), which operates by reflecting light of a wavelength defined by the pitch of helix-shaped LC molecules.⁴ CTLC displays have been used in the Matsushita Sigma Book and the Kolin i-library, but the appearance of their images is not as highly independent of viewing angle and lighting conditions as are images on paper [S-C. Yeng et al., Proc. IDW '04, 1527–1530 (2004) provide measurements on the performance of these two products as well as Sony's electrophoretic-display-based LIBRIé]. For these reasons, liquid crystals may not be the ideal technology for electronic-paper displays.

Other Technologies for e-Books

The following technologies meet the readability, portability, and ultra-low-power requirements of e-paper displays. While these technologies have their individual strengths, they are all intrinsically suitable for applications such as e-books because they do not require the use of polarized light and therefore exhibit brightness levels approaching that of paper.

Moving-Particle Systems. This class of electronic-paper systems comprises tech-nologies in which colored particles in a fluid are manipulated by electric fields. There are two basic sub-classes: electrophoretic systems, such as those of E Ink Corp. and SiPix Imaging, in which the particles are displaced,^5,6 and those such as Gyricon's in which the particles rotate.⁷

Electrophoretic (EP) displays operate by the motion of charged pigment particles in response to an electric field. The particle displacement is proportional to the time integral of the applied voltage. The two basic systems used involve particles of a single charge and color in a colored fluid (SiPix Imaging)⁶ or a dual-particle system (E Ink Corp.).⁵

The E Ink Corp. front plane (winner of the 2004 SID/Information Display Display Material or Component of the Year Gold Award) consists of microcapsules containing a liquid and two types of tiny pigment particles: black ones that are for the purposes of this description negatively charged and white ones that are positively charged (the combination of charge and color is arbitrary). The micro-capsules are laid down as a layer in making a display. If a positive voltage is applied to a bottom electrode under the microcapsules (relative to the transparent top electrode), the positive particles will migrate to the top and produce a white image, while the black particles will migrate to the bottom. When the white particles are on the viewing side of the cell, the display appears light from any viewing angle because incident light is scattered from the white particles.

Intermediate gray states, in which the black and white particles are partially intermingled in the body of the capsule, may be produced by applying intermediate voltages for a fixed time or by applying the original voltage for a shorter time. A significant issue in conventional moving-particle systems is that they have a slow response time.

Electrochromic Systems. Electrochromic systems have been long established and rely upon a color change produced in a material by an electrically induced oxidation or reduction of the material.⁸ Well-known electrochromic systems are the viologen molecules and oxides of tungsten. Electrochromic systems require a high current to change images and have previously suffered from limited material stability, although stability has now been considerably improved.

Electrodeposition Systems. Electrodeposition, as reported by Sony, is similar to electro-chromism in that an electrical current induces a change in the optical state.⁹ But in this case, material is physically electrodeposited onto an electrode from an electrolyte solution in order to induce the optical change.

Electrowetting Systems. The electrowetting technology developed by R. A. Hayes and B. J. Feenstra¹⁰ has been described in a recent article by J. C. Heikenfeld and A. J. Steckl in Information Display 25, No. 11, 26–31 (November 2004). Here, an electric field was used to move a colored oil across a surface. In equilibrium, the colored oil naturally forms a stable and continuous film between a hydrophobic insulator and a layer of water that covers the oil and acts as an electrode. However, when a voltage difference is applied across the hydrophobic insulator, an electrostatic term is added to the energy balance and the stacked state is no longer energetically favorable. The system lowers its energy by moving the water into contact with the insulator, thereby displacing the oil and exposing the underlying white surface. The balance between electrostatic and capillary forces determines how far the oil is moved to the side.

 Philips Research Laboratories

Fig. 1: This high-resolution active-matrix 1-in. electrowetting demonstration cell was described at IDW '04 in Niigata, Japan. There are 105 rows and 105 columns, for more than 11,000 pixels.

Electrowetting can provide an optical switch with high reflectivity (greater than 40%) and high contrast (greater than 15:1). In addition to the excellent optical properties, the technology exhibits a video-rate response speed of about 10 msec, has a clear route toward a high-brightness color display, and is readily scalable to a pixel size of 160 μm, as was recently demonstrated in an active-matrix electrowetting display (Fig. 1). A problem with electrowetting systems is their lack of bistability.

Microelectromechanical Systems (MEMS). This class of e-books is comprised of technologies in which micro-structured foils are either deformed, as in the Iridigm display,¹⁶ or rolled up¹¹ by applying an electrical field, thereby modulating the reflectivity of the system.

The deflected-foil technologies operate by interference modulation and switching between two modes: (1) a resonance mode, in which a reflective color is generated by interference in an air gap between the reflective foil and the substrate and (2) an absorption mode, in which the foil is directly in contact with an absorbing thin-film stack on the substrate. This approach produces binary devices with a color defined by the size of the air gap.

Operation of the rolled-foil technologies is conceptually simple. The foil acts as an old-fashioned window blind, which can either block or transmit light, depending upon how far it is unrolled.

Toward Commercial e-Books

Table 1 summarizes the properties of the high-brightness technologies which are suitable for electronic-paper applications. Although several high-brightness technologies offer the readability, portability, and ultra-low power consumption required for e-books, the moving-particle systems are already establishing themselves in the electronic-paper market for both e-book products and signage applications. Therefore, for the remainder of this article we will concentrate upon e-books using the electrophoretic approach.

A commercial e-book display using micro-encapsulated electrophoretic electronic ink, with a real paper-like look, has been realized by Philips, E Ink Corp., and Toppan Printing Co. (Table 2).¹²

Sony's LIBRIé e-book reader, the first device to utilize the Philips display solution for enhanced reading and winner of the 2004 SID/Information Display Display Product of the Year Gold Award, is similar in size and design to a paperback book (Fig. 2). The LIBRIé allows users to download published content, such as books or comic strips from the Internet, and can store up to 500 downloaded books.

 Sony/Philips

Fig. 2: Sony's LIBRIé e-book reader is the first commercial product to use the Philips/E Ink Corp. active-matrix electrophoretic display. The 800 x 600 6-in. display exhibits four gray levels.

Table 2: Characteristics of the Commercial Active-Matrix Electrophoretic Display

The LIBRIé e-book's perceived image quality improves as gray levels are added, so a series of driving pulses were generated to switch the display between four gray levels – white, black, and two intermediate gray states. Since each of the display's states are stable, allowing images to remain on the display when power is off, an important part of the driving scheme is a look-up table through which the driving circuitry can determine the proper voltage for driving each pixel of the display from its existing level to the desired new one. However, this process is sufficiently imprecise so that "ghosts" of the previous image appear on the new image.

A clearing step that solves this problem, and is fairly quick and undemanding of resources, has been developed. The display is first reset to one of the two extreme black or white "rail" optical states to which the gray tones will be added. The gray-scale accuracy is ensured by the fact that the rails are the most reproducible reference states. Resetting towards a rail in a single step, rather than multiple steps, minimizes flicker and shortens the update time.

In one rail-stabilized driving approach based on reset to the closest rail, the reset state is determined by the content of the next image regardless of the previous image, and the gray levels are always achieved by way of the closest rail (Fig. 3). To achieve a light-gray state between white and middle gray, the white state is selected as the reset state because it is the closest to the light gray. To achieve a dark-gray state between black and middle gray, the black state is selected as the reset state because it is the closest to the dark-gray state.

Thus, in an image update, a 1-bit version of the new image is obtained first, immediately followed by a gradual addition of the gray tones from their respectively closest rail states. Accurate gray scale is achieved with minimal flicker and shortened update time.

Fig. 3: To prevent the appearance of ghosts of the previous image on the next one, one approach is to reset each pixel by driving it to the extreme state (black or white) closest to the level it will have in the image to come. The gray levels are always achieved by way of the closest extreme state, or "rail."

 Polymer Vision

Fig. 4: One of the most attractive features of paper is its flexibility. Polymer Vision's approach to a flexible electronic-paper prod-uct combines a plastic substrate and an electro-phoretic front plane.

Into the Future

While today's electronic paper can achieve the look of a black-and-white book, there are clear benefits associated with introducing colored images. One approach to color is to place a color filter, as used in color LCDs, on top of the electronic paper. Unfortunately, this reduces the brightness of the e-book.

A second approach is to introduce intrinsically colored components into the electronic paper. These components can be colored liquids, as used by SiPix Imaging and in the electrowetting display^6,10; colored particles, as used by E Ink Corp.¹⁴; or multi-colored electrochromic materials.¹⁵ Alternatively, a technology can be chosen that is based upon optical-interference effects, as employed in cholesteric LCDs and in the Iridigm display.

One of the most attractive features of paper is its flexibility. Several approaches have been used to create a flexible electronic-paper product using a plastic substrate.¹⁷ Electro-phoretic systems are particularly amenable to the production of a flexible electronic-paper display (Fig. 4).¹⁸

Finally, there is always the dream that electronic paper will provide options that real paper cannot. For example, we have seen the development of a pen-input system by Philips and E Ink Corp. to create an electronic drawing pad.²²

Perhaps a more-attractive feature would be moving-picture capability, which can be realized by those electronic-paper technologies with a response time below 50 msec. Some technologies, such as electrowetting and the MEMS-based systems, are intrinsically fast enough. But we have recently seen that some traditionally slow technologies have been speeded up to approach video speeds. NTERA, Ltd., has demonstrated an electro-chromic system with fast switching,²⁰ while moving-particle systems have been brought into the video realm by developments from E Ink Corp. and Sunnybrook.²¹

Perhaps the most spectacular step towards an ultra-high-speed electronic-paper display has been the electrophoretic approach taken by the Bridgestone Corp.; their approach involves the movement of black and white charged particles in air.¹⁹ Because of the low viscosity of air, the response time of this electronic-paper technology is about 0.2 msec, approximately 100 times faster than that of an LCD!

Conclusions

Although LCDs are not ideal for electronic-paper applications, they were used a few years ago – before alternatives were mature enough for commercial use – in an unsuccessful wave of e-book introductions. Now, a range of new technologies is approaching the e-book requirements of readability, portability, and low power consumption.

Electronic-paper products based upon electrophoretic technology are already appearing. The accurate gray-scale reproduction and extremely low power consumption of these products, together with high brightness and a nearly perfect viewing angle, combine to give these displays their unique paper-like properties. The future of e-books looks ever more colorful, flexible, dynamic, and interactive.

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