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  • 19:00 06 June 2001
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  • Duncan Graham-Rowe
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Photo: Nick Cunard/PYMCA
Photo: Nick Cunard/PYMCA
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The day you can download New Scientist magazine onto "electronic paper" and take it anywhere to read just got a step closer. E Ink of Boston announced this week that it has succeeded in making electronic paper work in full colour.

Like ordinary paper, electronic paper works entirely by reflection. This means that, unlike competing electronic displays like LCDs, it never needs a backlight. In addition, it only needs power when the image changes. Once an image has been produced it will remain visible even with the power switched off.

Laptops, palmtops and cellphones with rigid electronic paper screens will be on the market within the next two years, says E Ink's Dan Button, who demonstrated the new colour display this week at the Society for Information Display conference in San Jose, California.

The new display is based on E Ink's monochrome e-paper, which consists of millions of transparent microcapsules sandwiched in a thin layer between two arrays of electrodes. The array corresponding to the surface of the paper is transparent (New Scientist, 15 May 1999, p 36).

Quick switch

Each tiny capsule contains white granules suspended in a dark, oily liquid. When an electrode in the upper surface is given a negative charge, it attracts granules towards it, making the surface appear white. Reverse the polarity and the granules are pulled to the bottom, revealing the dark liquid and making the surface appear black. The spaces between electrodes are small enough to give a resolution of 300 monochrome dots per inch (dpi).

To create a full colour display they laid a fine coloured filter across the top of the monochrome display - the same trick that lends colour to LCDs. The firm admits it's not an elegant approach.

"This route gets us on the market quickly, since it uses technology that already exists," explains Button. E Ink developed the colour technology with Japanese printing company Toppan, which makes transparent colour filters for LCD displays.

The filter makes each pixel appear either red, green or blue when the pixel below it is white. When the pixel is black, the filter above reflects very little light so no colour is seen. "We are either exposing a highly reflective coloured surface or a black surface," says Button.

Flexible friend

Eventually, they hope e-paper will be flexible enough to be a paper substitute. Meanwhile, E Ink expects it to rival liquid crystal displays and the emerging organic LED displays (New Scientist, 21 October 2000, p 48).

The firm's next challenge is to improve the resolution of the colour display. A drawback of the filter approach to colour generation is that the filters need a single pixel for each primary colour. This effectively reduces the resolution by about a third, to 80 dpi.

This shouldn't be a major problem, says Button, since the resolution is not so much determined by the size of the microcapsules as by the size of the electrodes and filters.

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