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Optoelectronics/Displays??

Enabling e-paper displays

Posted: 01 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:E-paper? reflective-display technologies? electrochromic and electrophoretic displays? ChLCDs? TN LCDs?

We've all heard the sizzle of e-paper displays, with their promise of wraparound advertising signs, wearable wrist monitors and e-newspapers that we fold or roll up and stick into our pockets. While the age of these gee-whiz products is not yet upon us, their enabling display technologies are on the move.

E-paper comprises a number of reflective-display technologies, which break down roughly into particle displays, LCDs and MEMS displays. The particle displays include electrochromic and electrophoretic displays (ECDs and EPDs); the LCDs are mainly cholesteric devices (ChLCDs) and proprietary bistable twisted-nematic (TN) LCDs.

Major vendors in this arena include Kent Displays Inc. and Fujitsu Frontech Ltd in ChLCDs; E Ink Corp. and SiPix Imaging Inc. with EPDs; Nemoptic and ZBD Display Inc. in bistable TN LCDs; and Qualcomm with its IMod (Interferometric Modulator) MEMS display. Varitronix Ltd makes both ChLCDs and bistable TN LCDs.

Almost all the e-paper technologies are relatively immature and at the beginning of their evolution of capabilities. The different e-paper technologies have different native strengths and weaknesses, but they all share good contrast, low-power operation and bistability, which means no electrical refresh is required once a display screen is drawn.

The upshot, to cite just one application example, is that the owner of a Sony Librie e-book, based on an E Ink EPD, can expect to read about 7,000 pages of text before having to think about a recharge, while the owner of a two-page Panasonic Sigmabook, based on Kent ChLCDs, will get three to six months of operation from the two AA batteries stashed in the spine of that e-book.

Electrophoretic products
The most visible of the e-paper displays has been E Ink's EPD, which has appeared in e-books, tablet computers and watches, and Motorola's unique Motofone cellphone. All of these products use monochrome EPDsthe former group with a glass-substrate, active-matrix display in a conventional rectangular format, the latter group using flexible, irregularly shaped, passively addressed displays built on plastic.

E Ink has fielded its second generation of EPD. The new displays have improved reflectivity by about 25 percent, speed by about 50 percent and gray scaling by 100 percent over the previous generation. The company has prototyped (but not fielded) various color displays and has demonstrated multicolor EPDs using color filters.

SiPix Imaging Inc. is also fielding passive, segmented EPDs on plastic and active-matrix dot-matrix EPDs on glass.

The company's Microcup EPD displays are named for the plastic partitions that enclose the display material within sealed cells. The first products in which the displays are appearing are standalone, segmented pricing signs, measuring about 127mm x 178 mm and 178mm x 254mm, for supermarket meat and produce sections.

This message board requires no power to maintain screen contents.

SiPix claims about twice the contrast of comparable TN LCDs for these flexible displays, along with a wide viewing angle and, of course, the power savings that bistability brings. By using different color particles, the company offers a combination of white plus red, green, blue, gold or black.

Anchored LCDs
Another prominent e-paper type is the bistable TN LCD, which differs from conventional LCDs in one critical detail: a proprietary orientation layer that anchors the LC molecules on one substrate. One example is the grating-aligned zenithal displays of ZBD Displays.

Electrophoretic displays like these pricing signs come in passively addressed segmented varieties (shown) and active-matrix dot-matrix varieties.

ZBD is fielding a 2.85-inch-diagonal, 0.25VGA format, monochrome shelf label on glass that claims three times the contrast of a conventional LCD, and higher speed than any other bistable display. In environments where the display is updated five times a day, it will operate from a single coin battery for about five years, the company said. An A5 size (148mm x 210mm) product is in the offing. Color is not a current priority.

Creating textures
ChLCDs are the one somewhat mature technology in the world of e-paper displays, available from Kent since the mid-1990s. With ChLCDs, the electrically controllable alignment of the liquid-crystal molecules creates "textures" with different optical characteristics, all bistable. The technology offers excellent contrast and viewing angle, plus substantial power savings.

Kent's lineup includes a blue/white display and displays with black plus yellow, green or yellow/green. Multicolor ChLCDs, which contain stacked monochrome displays of primary colors, are also available. These displays are all glass-based, but the company expects to have flexible versions, built on plastic substrates, available by the middle of the year.

For its line of ChLCDs, Varitronix has developed its own textures, offering a choice of blue/white, red/amber, yellow/green, pink/peach and orange/yellow.

As for Fujitsu Frontech, that company is stacking ChLCDs to provide multicolor displays for the FLEPia handheld reader and tablet that went into field trials last year. A fully charged FLEP is said to provide about 50hrs of service.

- David Lieberman
EE Times




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