E-paper rivals mimic gems and butterflies

Screens on the current crop of e-book readers fall far short of the ideal of a display that combines the advantages of paper with the speed and vivid interactivity of an LCD.

They are paper-like in that a static image draws no power and they can be read by the light of a lamp or the sun, so their batteries can last days between charges. But they are slow to refresh, they lack the contrast of paper, and most of them do not offer colour.

So how good are they going to get? E-ink, which makes the e-paper layer on the screens of almost all e-readers on the market, will put a colour screen into mass production next year, according to its chief technology officer Lawrence Schwartz.

Its e-paper consists of a layer of microcapsules containing oppositely-charged white and black particles that can be moved in and out of visibility by an active matrix similar to those used in LCD screens. Colour is achieved by adding a mesh of filters.

Schwartz sees a big emerging market in schools, where the argument for e-textbooks is compelling: they save money, they can be updated easily, they are robust, and they mean children don't have to lug round heavy books.

Ohio-based Kent Displays has been first to market with a colour screen - the ChLCD display used on Fujitsu's Flepia e-reader. This uses a layer of microcapsules containing cholesteric liquid crystal, which has a helical structure that can be rendered transparent or reflective by switching an applied voltage. The UK company ZBD makes signage products using similar technology.

Colour pixels are produced by using different pitches of the helix. Kent's chief technology officer Asad Khan said the RGB crystals of each colour pixel can be stacked vertically or placed side by side - the latter arrangement being less efficient  but easier to make.

The Kent display is bistable like all those on e-readers, drawing power only when the image changes. Astonishingly, Kent has produced a sample screen that under some usage patterns can be a net generator of power; it uses a backplane of solar cells that capture the energy of the light that is not reflected by the screen.

Encapsulation of the liquid crystal allows Kent screens to be put on to a flexible substrate, making them more robust. Kent can even put a ChLCD coating on a mobile phone, allowing its colour to be changed to match whatever the user is wearing.

Schwartz and Khan discussed the future of e-reader screens at EmTech in Boston last week with representatives of two other companies offering rival display technologies - both a straight crib from nature.

Andre Arsenaut, chief technology officer of Opalux, explained that his P-ink technology takes its inspiration from the way opals produce colour reflections. "An opal is just glass and water. It is still amazing to me that two colourless materials can produce such bright colours," he said.

The effect stems from the gem's microstructure in which arrays of tiny spheres reflect different colours as the angle of the incident light changes. Opalux mimics this in the form of a polymer containing ordered layers of microscopic pores.

For some uses, such as for making hard-to-forge banknotes, the polymer can be pressure sensitive so that it changes colour when flexed. For displays it can be made electrosensitive, swelling slightly and changing colour with an applied voltage.

It is bistable, flexible, easily manufactured, and draws only microamps when switching. But Arsenaut freely admitted that the material, while having a variety  of uses, has problems when used as e-paper: blacks, whites and fine gradations are hard to achieve, switching is slow, and it is sensitive to viewing angle.

The fourth featured technology, from Qualcomm MEMs Technology (QMT), mimics the way colour is produced by tiny gaps on butterfly wings. "An optical engineer would call those spaces an optical resonator cavity," said business development officer Jim Cathey. "When ambient light enters that cavity, light of a certain colour is produced by a process of constructive and destructive interference."

Pixels in QMT's Mirasol screens each have cavities tuned to produce red, green or blue light, controlled by micromechanical shutters. They are described as bistable, and so should draw power only when changing, yet the drain of a 2.2in QVGA static image is quoted mysteriously as one milliwatt. QMT, a division of mobile chip giant Qualcomm, has promised to elucidate this point.

However QMT is playing a different game from the other e-paper specialists. The problem they face, argues Cathey, is that people have become accustomed to the far better speed, contrast and colour depth of LCD screens - and that if e-books are to mature as a publishing platform, they will need screens approaching this quality. He said: "To get performance you are always going to have make a trade-off with power [consumption]."

Cathey points out that the frugality of current e-paper screens is deceptive. Their battery life is measured in page turns rather than time "and every time you scroll a page it counts as a page turn...If you tried to do video, where the image is changing all the time, these screens might actually consume more power than an LCD."

E-Ink's Schwarz conceded that his e-paper will not support video but he said a lot can be done with animation, which would not usually require a full-page refresh per frame. "We expect to see animation starting from next year," he said.

QMT, which will open a new Mirasol fab next year, has launched small mobile-phone screens with a low colour depth and supporting 15 frames-per-second video. "These are very early days in the commercialisation of our technology," said Cathey.

His company measures its products not against static e-paper but against LCDs, which it says draw up to twenty times the power. Cathey is surely right in one respect: if any company succeeds in approaching LCD quality at a much lower power drain, the distinction between e-readers and more versatile mobile products will disappear. µ