A REVOLUTIONARY display technology finally promises to make high-resolution,
widescreen televisions more affordable. The display, developed at Philips鈥檚
research lab in Eindhoven, the Netherlands, uses a rotating prism to create a
large, full-colour image from a single, cheap liquid-crystal display chip.
The new screen costs about half as much as a conventional LCD or plasma
screen, and consumes much less power than a bulky cathode-ray tube.
Display designers have to find their way through a maze of compromises. The
size of CRT screens is limited by the weight of the glass needed to make sure
that the evacuated tube doesn鈥檛 implode under atmospheric pressure, and its
electron guns waste energy as heat. But making large LCD panels is tricky
because a single faulty cell creates a permanent blip on the screen. Plasma
panel TVs are bulky, gobble power and cost over $10,000.
Advertisement
One way around these problems has been to use a back-projection TV that beams
pictures onto a translucent screen. In some projectors, red, green and blue
light passes through separate small LCD panels, which act as 鈥渓ight valves鈥�.
These three images are projected onto the screen, where they are superimposed.
But it鈥檚 difficult to keep the colours perfectly aligned, and you have to pay
for three LCDs. In addition, the layer of transistors which control the LCD鈥檚
cells block some of the light, reducing brightness.
Philips鈥檚 answer is to use a new kind of LCD panel made by depositing a
liquid-crystal array directly onto a transparent silicon chip. It places a
transparent electrode sheet on top of the LCD and a reflective layer underneath.
Light shines through the top sheet and then through the LCD, before being
reflected by the backing layer into a projection lens system. The projected
image is very bright because there are no transistors to obstruct the light. The
resolution of Philips鈥檚 鈥渓iquid crystal on silicon鈥� (LCOS) panel is high because
the LCD cells can be very small鈥攋ust 2 micrometres apart.
To cut the cost of flat-screen/widescreen TV鈥攁nd its
complexity鈥攖he researchers have devised a clever way to squeeze
full-colour video from a single LCOS chip
(see Graphic). White light from an arc
lamp is split into red, green and blue beams, which pass through a rapidly
rotating prism. This continuously scrolls red, green and blue horizontal strips
of light down the LCOS panel, so the cells in the LCOS panel reflect the three
colours in sequence.
The video signal is split into red, green and blue frames and fed to the
panel in synchrony with the corresponding light beams. So when the LCD cells are
bathed in red light they display only the red content of the picture, when
bathed in blue light they display only the blue content, and so on. The prism
scrolls at 200 hertz鈥攆ast enough to fool the eye into seeing full colour
without flicker.
Project leader Ad de Vaan will demonstrate the new screen at the Society for
Information Display annual conference in San Jose, California, on 3 June. He
says 1-metre diagonal displays will be ready by 2003 and cost under
$3000.