The colourful promises of electronic paper
WITH the international launch of Amazon's Kindle reader this week, perhaps electronic books will become the must-have present this year. But even as we unwrap our shiny new e-readers, we may be forgiven for wondering how long it will be before the long-promised colour versions are available. Moreover, with multifunctional devices such as the iPhone becoming the norm, how soon could these e-readers make the breakthrough to display video?
Currently, the leading e-readers use proprietary "electrophoretic" display technology from a firm called E-Ink - a 12-year-old spin-off from the Massachusetts Institute of Technology. While this approach has paved the way for monochrome displays, it is struggling to move beyond them. E-Ink has yet to deliver on its promise of colour displays that retain the fine resolution of its monochrome ones, never mind video.
E-Ink has yet to deliver colour displays that retain the fine resolution of its monochrome ones So could other e-paper displays achieve good-quality colour and video while also maintaining the low energy demands and high resolution of monochrome electrophoretic displays?
There are several contenders out there. The latest approach, and perhaps the most promising, uses photonic crystals (see "The fluid world of e-paper"). It has the potential to rival traditional LCD and plasma screens in terms of colour quality because it enables entire pixels to be tuned to specific wavelengths of light - in some cases, at eye-catching speeds.
Opalux, a spin-off company from the University of Toronto, Canada, which has created Photonic Ink (P-Ink), says the advantage of tunable pixels is high colour intensity. For example, colour LCDs require red, green and blue sub-pixels in order to produce full colour. So if you want to display, say, pure red, then at best only one-third of the display will actually appear red. Not so if every pixel can be individually tuned to reflect a specific colour.
In contrast, E-Ink's approach to colour has simply been to colour white pixels with filters. This means that the resolution suffers - because what would count as a pixel in monochrome displays now has to act like a sub-pixel. Furthermore, filters tend to degrade the quality of displays: "If you start to use filters you're throwing away photons," explains Edwin Thomas, who has developed chemically tunable photonic crystals at MIT.
Opalux is already working with display firms to produce next-generation prototypes, says its head of technology, Andre Arsenault. But, he says, this is a long-term endeavour. In the short term, the company is focused on more niche applications such as battery-level indicators.
Opalux is not yet able to retune its P-Ink pixels fast enough to produce video, but there are other photonic-crystal systems that can, such as that developed by David Snoswell, a former researcher at Kodak Research UK and now at the University of Cambridge. "We were getting sub-millisecond switching rates," he says.
Frustratingly for Snoswell, he was forced to abandon this work when Kodak closed its UK research facility earlier this year. Even so, the research demonstrates that photonic crystals are potentially fast enough to create "field-sequential" colour displays, says Snoswell, in which full colour is created by flashing red, green and blue pulses in a single pixel one after the other.
Meanwhile, the fastest monochrome pixel-switching that E-Ink is willing to talk about having achieved is 50 milliseconds, the equivalent of 20 frames per second - still too slow for video.
Nevertheless, E-Ink insists that it is on track to meet the future demands of its customers. "Right now I can't see any particular technology gaining momentum," says E-Ink's vice-president of marketing, Sri Peruvemba. He says the company is aiming to bring its colour displays to market by 2011, with video to follow.
With others virtually certain to follow them, it looks for the first time as though E-Ink will face some colourful competition.
The fluid world of e-paper
It's a trick that has long been used by nature to create iridescence in opals and the vivid colours in chameleons' skins. Arrange microscopic particles in periodic rows and, depending on their size and spacing, you can start to influence the way light is reflected off a surface.
For visual displays, these periodic structures, known as photonic crystals, can be made of pretty much anything, from tiny silica beads or liquid crystals to polystyrene balls - or just voids in a substrate.
Provided the particles are about 200 nanometres in diameter, their interval-based structure will cause an effect known as Bragg interference at visible wavelengths, which influences the apparent colour of their surface.
To make this into a display you must be able to tune the crystals by altering the size of the spaces between their particles. There are many ways to do this. Opalux is a firm based in Toronto, Canada, whose Photonic Ink (P-Ink) uses silica beads embedded in an electroactive polymer substrate. When an external electric field is applied, the substrate can be made to swell or contract, altering its colour.
Other techniques include the use of self-assembling copolymers that swell in response to chemicals. But perhaps most promising, in terms of producing high-quality colour displays with pixel-switching rates fast enough for use in video, is a technique developed by Kodak based on the principles of dielectrophoresis. It use electric fields to manipulate particles suspended in a fluid, much like E-Ink's electrophoretic approach (see main story). However, instead of using the field to move the particles to the surface of the fluid, or away from it, this approach uses the field to control the spacing of the particles in photonic crystals.
"You induce an electric dipole in the particles with an electric field," says David Snoswell, who worked on the Kodak project. "Like little bar magnets, they all line up."
As with E-Ink's displays, the viscosity of the fluid can slow the rate at which the particles move, limiting the pixels' switching rate. But because the tuning process requires the particles to move much shorter distances, they can switch in sub-microsecond times - which is more than adequate for video.
