products enabling the worldâ„¢s smallest data and video projectors, HDTVâ„¢s, and digital cinema, DLP technology is extremely powerful and flexible. With success of the DMD as a spatial light modulator for projector applications, dozens of new applications are now being enabled by general- use DMD products that are recently available to developers. The same light switching speed and on-off (contrast) ratio that have resulted in superior projector performance, along with the capability of operation outside the visible spectrum, make the DMD very attractive for many applications, including volumetric display, holographic data storage, lithography, scientific instrumentation, and medical imaging
The digital micromirror device, or DMD, is a micro-opto-electromechanical (MOEMS) system that is the core of Texas Instruments (TI) DLP technology. DMD was invented by solid-state physicist Dr. Emmanuel Larry Hornbeck in 1987. The DMD project started as the Deformable Mirror Device in 1977 using analogue micromechanical light modulators. The first DMD analog product was the TI DMD2000 airplane ticket printer that uses a DMD instead of a laser scanner.
A DMD chip has on its surface several hundred thousand microscopic mirrors arranged in a rectangular array that correspond to the pixels of the image to be displayed. The mirrors can be rotated individually ± 10-12 °, to a state of on or off. In the power-on state, the projector's light bulb reflects off the lens causing the pixel to appear bright on the screen. In the off state, the light is directed elsewhere (usually over a heat sink), making the pixel appear dark.
To produce gray scales, the mirror turns on and off very quickly, and the ratio of activation time to shutdown time determines the shadow produced (binary pulse width modulation). Contemporary DMD chips can produce up to 1024 shades of gray (10 bits). See Digital Light Processing to see how color images are produced on DMD-based systems.
The mirrors themselves are made of aluminum and measure around 16 micrometers. Each is mounted on a yoke which in turn is connected to two support posts by conforming twist hinges. In this type of hinge, the shaft is fixed at both ends and twisted in the center. Because of the small scale, hinge fatigue is not a problem and tests have shown that even 1 trillion (1012) operations do not cause noticeable damage. Tests have also shown that hinges can not be damaged by normal shock and vibration as it is absorbed by the DMD superstructure.
Two pairs of electrodes control the position of the mirror by electrostatic attraction. Each pair has an electrode on each side of the hinge, with one of the pairs arranged to act on the yoke and the other acting directly on the mirror. Most of the time, equal bias loads are applied to both sides simultaneously. Instead of turning to a central position as one might expect, this actually keeps the mirror in its current position. This is because the pulling force on the side at which the mirror is already tilted is greater, since that side is closer to the electrodes.
To move the mirrors, the required state is first loaded into an SRAM cell located below each pixel, which is also connected to the electrodes. Once all the SRAM cells have been charged, the polarization voltage is removed, allowing the SRAM cell loads to prevail, by moving the mirror. When the bias is restored, the mirror is held once more in position, and the next required movement can be loaded into the memory cell.
The polarization system is used because it reduces the voltage levels required to direct the pixels so that they can be driven directly from the SRAM cell, and also because the bias voltage can be eliminated at the same time for the chip as a whole, so that each mirror moves at the same instant. The advantages of the latter are more accurate synchronization and a more cinematic mobile image.