Digital micromirror device

The digital micromirror device, or DMD, is a micro-opto-electromechanical system (MOEMS) that is the core of the trademarked DLP projection technology from Texas Instruments (TI). The DMD was invented by solid state physicist and TI Fellow Emeritus Dr. Larry Hornbeck in 1987.

The DMD project began as the Deformable Mirror Device in 1977 using micromechanical analog light modulators. The first analog DMD product was the TI DMD2000 airline ticket printer that used a DMD instead of a laser scanner.

A DMD chip has on its surface several hundred thousand microscopic mirrors arranged in a rectangular array which correspond to the pixels in the image to be displayed. The mirrors can be individually rotated ±10-12°, to an on or off state. In the on state, light from the projector bulb is reflected into the lens making the pixel appear bright on the screen. In the off state, the light is directed elsewhere (usually onto a heatsink), making the pixel appear dark.

To produce greyscales, the mirror is toggled on and off very quickly, and the ratio of on time to off time determines the shade produced (binary pulse-width modulation). Contemporary DMD chips can produce up to 1024 shades of gray (10 bits). See Digital Light Processing for discussion of how color images are produced in DMD-based systems.

The mirrors themselves are made out of aluminum and are around 16 micrometers across. Each one is mounted on a yoke which in turn is connected to two support posts by compliant torsion hinges. In this type of hinge, the axle is fixed at both ends and twists in the middle. 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 the hinges cannot be damaged by normal shock and vibration, since it is absorbed by the DMD superstructure.

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