Electronic paper reaches video speed
Colour movies might soon be playing on single sheets.
25 September 2003
PHILIP BALL
One display device could hold an entire library.
© Philips Electronics
Paper capable of playing videos has been invented at the Philips Research laboratory in Eindhoven, the Netherlands1.
A single sheet looks pretty much like ordinary paper. But the ink can be rearranged electronically fast enough to show video movies.
Its devisers, Robert Hayes and Johan Feenstra, have also figured out how to create full-colour displays. Their colour screens would be four times brighter than the flat devices currently made from liquid crystals, they reckon.
The invention is the latest version of 'electronic ink'. Researchers hope to combine the convenience, robustness and readability of printed material with the vast and flexible information content of laptop computers.
In principle, a plastic sheet covered with electronic ink could display an entire library, page by page. The information would be stored in a portable chip, and the display would be powered by a slimline, lightweight battery. Harry Potter and the Order of the Phoenix would weigh no more than a feather.
Switch in time
High-resolution monochrome electronic paper is already on the verge of commercialization, produced by Massachusetts-based company, E-Ink, in collaboration with Philips. Here the 'ink' consists of countless tiny, transparent capsules of black and white powdered pigments, which are drawn by electric fields to the front face.
This system is fine for viewing successive pages of a book, but its switching time is too slow for moving pictures. Hayes and Feenstra switch the colour of their e-ink in a completely different way.
Each pixel of the new display contains a drop of coloured, oily ink that spreads over a reflective white background. The white backing is coated first with a transparent material that conducts electricity - permitting electrical control of the pixel colour - and then with a transparent film of a water-repellent plastic.
Left to its own devices, the ink droplet spreads across the entire pixel. If a voltage is applied, it retracts like a bead of water in a Teflon pan, exposing the white area below. If the pixel is small enough, these white and inky regions are not visible, just an average brightness. When the droplet is fully spread, the pixel looks dark. When it retracts, the pixel looks much lighter.
Full-colour displays can be made with three sub-pixels of yellow, cyan and magenta.
© Hayes & Feenstra
The larger the applied voltage, the more the ink retracts. The ink is therefore capable of a continuous grey scale, not just of a two-tone contrast. So monochrome images can look very smooth.
The key to the system's success is its switching voltage. It is low enough that controlling the electronic ink requires only a small power source. Switching between dark and bright states takes only about ten milliseconds - fast enough to produce sharp video images.
In principle full-colour images might be produced this way, Hayes and Feenstra show. Pixels can be composed of three sub-pixels inked with the standard yellow-cyan-magenta tricolour system.
