A cell phone that can project a high-definition television image could soon be possible, say researchers at Cornell University who have developed a new microelectromechanical system (MEMS) for rapidly scanning wide areas with a laser. A projector based on the device would be about the size of dime and could cast a meter-wide image on a surface only half a meter away.
The key is a small mirror, about half a millimeter across, suspended by carbon fibers – rolled-up sheets of crystalline carbon commonly used to reinforce materials. The fibers amplify the vibrations of a piezoelectric motor, moving the mirror. This movement deflects a laser at different angles, causing it to sweep back and forth across a surface. While the current device only moves the laser side to side, the researchers say it can be easily mounted on a stage that tilts up and down to allow the device to sequentially draw each line of an image, using complex electronics that turn the laser on and off as it is directed across the screen to create the light and dark pixels. A full-color display would mix light from red, green, and blue lasers.
MEMS-based displays already exist in commercial products. Texas Instruments, based in Dallas, TX, for example, has developed a chip that uses millions of tiny mirrors, each of which turns pixels on and off by either turning toward or away from a light source (see “May the Micro Force Be with You”). This chip is now used in a variety of televisions and movie projectors. Another company, Microvision, in Redmond, WA, uses a single mirror MEMS device more like the one being developed at Cornell, but without the carbon fibers. The company is developing a full-color display.
The Cornell researchers say what sets their device apart is the high scanning speed of the mirror, combined with its ability to scan over a wide angle. The wide angle of the system is made possible, says Michael Thompson, a materials science and engineering professor and one of the researchers on the project, because the carbon fibers can bend sharply without breaking, giving the mirror a wide-range of movement. The fibers are also very stiff, which allows them to spring back and forth very quickly. High-speed vibrations are essential to creating high-resolution images. The researchers report mirror vibration frequencies of 35,000 cycles per second – enough, they say, to scan an image with a resolution of about 1280 by 768 pixels about 60 times a second. They say this resolution is comparable to some high-definition televisions, although this refresh rate can – under some conditions – show a detectable flicker.
Ming Wu, an electrical engineering and computer science professor at the University of California, Berkeley, says that in addition to high scanning speeds, the resolution of an image depends on the size of the mirror used. In the past, he says, mirrors large enough to produce high-quality images, on the scale of a few millimeters across, have been a challenge: it’s difficult to make the mirrors vibrate fast enough without breaking the apparatus. Thompson says the tough carbon fibers have allowed them to use a mirror half a millimeter across, already about the size-scale needed. He adds that by using more carbon fibers, the Cornell researchers expect to be able to increase the size even more.