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.
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. 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 InkMovie Camera (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. |