THE PHYSICS
Balancing two torques
A voltage applied to the bottom electrode pulls on the mirror support, creating an electrostatic torque. The twisting torsion beam pushes back with a mechanical restoring torque. The mirror settles wherever those two torques balance.
PULL-IN
The moment it snaps
As voltage increases, electrostatic torque grows faster than the mechanical restoring torque can keep up with.
Past a critical voltage, the balance fails entirely and the mirror snaps the rest of the way to its landing position. This is called pull-in, and it's exactly why a DMD mirror behaves like a clean binary switch instead of a gradual dimmer.
AN ALTERNATIVE APPROACH
The Grating Light Valve
Instead of tilting a mirror, the Grating Light Valve (GLV) uses an array of electrostatically actuated diffraction-grating ribbons. Changing the spacing between ribbons changes which color of light is diffracted toward the viewer.
MIRROR VS. GRATING
Two ways to modulate light
Reflection (Mirror)
Broadband, 1 mirror per pixel, larger millisecond-scale displacement, voltage-controlled, constant intensity.
Diffraction (GLV)
Wavelength-dependent, about 6 ribbons per pixel, tiny quarter-wavelength nanosecond-scale displacement, diffracted intensity varied by voltage.
A THIRD APPROACH
Qualcomm's mirasol display
A mirasol display is, at its core, an optically resonant cavity. A self-supporting deformable reflective membrane and a thin-film stack each act as one mirror of that cavity, both sitting on a transparent substrate.
UNIT 23 STUDY COMPLETE
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You've covered Tilting Mirrors & Gratings.