21 July 2008--Researchers from Microsoft say they've built a prototype of a display screen using a technology that essentially mimics the optics in a telescope but at the scale of individual display pixels. The result is a display that is faster and more energy efficient than a liquid crystal display, or LCD, according to research reported yesterday in Nature Photonics.
Anna Pyayt led the research as part of her Ph.D. thesis at the University of Washington in collaboration with two Microsoft engineers. Microsoft funded the work and has also applied for a patent on the technology.
The most common display technology, the LCD, is inefficient. The display is lit from the back, and less than 10 percent of the light reaches the surface of the screen. Pixels in a display technology work as on-off shutters, but the light has to travel through several layers before reaching the screen. In an LCD, one of those layers is a polarizing filter, which absorbs about 50 percent of the light as it passes through.
By contrast, the telescopic pixel design uses reflective optics. Each pixel functions as a miniature telescope. There are two mirrors: a primary mirror facing the backlight (away from the screen) with a hole in the middle, and a smaller secondary mirror 175 micrometers behind the primary mirror it faces. The secondary mirror is the same size and shape as the hole. Without an electric field, the mirrors stay flat, and light coming from behind the pixel is reflected back, away from the screen. But applying voltage bends the primary mirror into the shape of a parabola. The bending focuses light onto the secondary mirror, which reflects it out through the hole in the primary mirror and onto the screen.
The design greatly increases the amount of backlight that reaches the screen. The researchers were able to get about 36 percent of the backlight out of a pixel, more than three times as much light as an LCD can deliver. But Microsoft senior research engineer Michael Sinclair says that through design improvements, he expects that number to go up--theoretically, as high as 75 percent.
The telescopic display can also switch its pixels on and off faster than an LCD can, going from dark to light and back again in just 1.5 milliseconds, about six times as fast as a typical LCD pixel.
Researchers not associated with the study also see promise in the technology, particularly because it does not compromise picture quality for power efficiency. ”This novel approach for transmissive displays is highly attractive because it can provide high-efficiency analog gray scale,” says Jason Heikenfeld, assistant professor of electrical engineering at the University of Cincinnati and director of the Novel Devices Laboratory. Most microelectromechanical systems display technologies--such as Texas Instruments' digital micromirror devices--are digital; they are either on or off. In the telescopic pixel design, the amount of light emitted is a function of voltage. The mirrors act like springs--when you apply more voltage, they bend further, reflecting more light to the screen.
Other reflective displays that have tried to improve efficiency have not been popular with consumers because they are not very bright, says Heikenfeld. So a technology like the telescopic pixel design may serve to satisfy demands for power efficiency and image quality, he adds. However, the telescopic pixels require high voltages to operate--up to 120 volts--and Heikenfeld believes that Microsoft will have to reduce that voltage for a commercial product.
One potential concern about the technology may be its durability, because of the constant bending and movement of the mirrors. A durability test has not yet been done, says Microsoft's Sinclair. However, he adds that the group did produce an array of pixels that performed without any glitches, a sign that the technology can be manufactured. ”It shows definite signs of a future,” he says.
Pyayt agrees. ”It's not a final, perfectly working system, but it's in progress, and I believe it's possible to optimize it to be fully functional,” she says.
The technology is still in its nascent stages, and the project is unusual for Microsoft, which is not in the display business. Sinclair says there is a possibility that Microsoft will collaborate with a display manufacturer, but commercial production is at least five years away.