A pixel that uses a pair of mirrors to block or transmit light could lead to displays that are faster, brighter, and more power efficient than liquid crystal displays (LCDs). Researchers at Microsoft Research who published their novel pixel design in Nature Photonics say that their design is also simpler and easier to fabricate, which should make it cheaper.
LCDs corner half of the global TV market and are the most popular technology for cell phones and flat-panel computer monitors. But for three reasons, they do not boast the best image quality. First, the pixels do not turn completely off. Second, it takes 25 to 40 milliseconds on average for the pixels to switch between black and white, which is slow enough to blur fast-moving images. Third, LCDs are almost impossible to use in bright ambient light. “There is nothing in LCD technology that stands out,” says Sriram Peruvemba, vice president of marketing at electronic-paper pioneer E Ink, based in Cambridge, MA. “The only reason it has done well is it’s the lowest price [flat-panel] display today.”
The new telescopic pixels switch completely off and on within 1.5 milliseconds. Michael Sinclair at Microsoft Research says that the ultrafast response time translates to simpler, low-cost color displays. In LCDs, a pixel is made of three subpixels–red, green, and blue–that are lit up simultaneously at different intensities to create, say, yellow. Each subpixel is controlled with a separate transistor circuit, which makes the circuits complex. Because the telescopic display switches so rapidly, you could put red, green, and blue light-emitting diodes behind each pixel, Sinclair says, and have them sequentially light up to create a color shade. “This would reduce the complexity and cost of today’s LCD,” he says.
The telescopic pixels are also significantly brighter. In an LCD, by the time light passes through the polarizing films, the liquid-crystal layer, and the color filters, only 5 to 10 percent of it comes out. The telescopic pixels, on the other hand, let about 36 percent of the light through. “I could get by with a less-powerful backlight, because the telescopic pixel is more efficient,” Sinclair says. The greater brightness would also make the display more visible in bright sunlight.
The new pixels use two tiny micromirrors to pass or block light. The first is a 100-micrometer-wide, 100-nanometer-thick aluminum disc with a hole in the center. The other mirror, also a thin aluminum film, is as big as the hole and placed directly in front of it. Light is projected on the disc-shaped mirror from behind the second mirror.
In the “off” state, both mirrors reflect light back to the source, so nothing comes out of the hole. In the “on” state, a voltage applied between the disc and a transparent electrode bends the disc toward the electrode. Now, light bounces off the disc toward the second mirror and then out through the hole.
Sinclair and his colleagues fabricate the pixels in a layered fashion similar to that of silicon chip fabrication. He says that the telescopic pixel design is simpler than the design of an LCD, with fewer layers, so the fabrication would require fewer steps. Right now, the researchers use indium titanium oxide, the industry standard for making transparent electrodes. But they suggest making the electrodes with an extremely thin, patterned aluminum layer that would be nearly transparent. This could simplify the display’s production process and decrease its cost even more.
The new pixel technology has advantages over current LCDs, says Peruvemba, but the mechanical parts might compromise robustness. “There are literally hundreds of thousands to millions of little shutterlike devices that have a mechanical movement,” he says. “In most devices, what fails first are the mechanical parts.”
While LCD and the new telescopic display transmit light from a backlight, others have come up with promising pixels that reflect ambient light. Qualcomm’s new display, which has MEMS-based pixels, is set to debut this year on three different cell phones. (See “E-Paper Displays Video.”) The company has also announced its first color screen for an MP3 player. Meanwhile, E Ink, which sells black-and-white e-paper displays, has now made color and video prototypes. (See “E-Paper Comes Alive.”) The e-paper technologies have a niche market: low-power screens for outdoor use.
These displays do not need a backlight, and their pixels do not need the constant refreshing required in an LCD, which slashes their power use. And the more light, the better the screens look. “We’re not competing with bright ambient light–we’re taking advantage of all that sunlight,” says Brian Gally, director of engineering at Qualcomm MEMS Technologies. “So it’s really analogous to paper.”
Sinclair says that Microsoft Research is targeting large, low-cost computer screens. That could be an IT worker’s dream. Instead of having a small desktop monitor on which you have to switch between windows, a techie could have a “whiteboard-sized thin screen” to work on, Sinclair says.
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