2510332 - February 2009
Overview of DMD Use in Projection Optical Systems
The DMD device is the heart of DLPTM projection systems. The device is a bistable spatial light modulator, consisting of an array of movable micromirrors functionally mounted over a CMOS memory cell. Each mirror is independently controlled by loading data into the memory cell below the mirror to steer reflected light, spatially mapping a pixel of video data to a pixel on a display. The data electrostatically controls the mirror’s tilt angle in a binary fashion, where the mirror states are either +X degrees (on) or -X degrees (off). For current devices, X is typically 12 degrees (nominal). Light reflected by the on mirrors then is passed through a projection lens and onto a screen. Light is reflected off to create a dark field, and defines the black-level floor for the image. Images are created by gray-scale modulation between on and off levels at a rate fast enough to be integrated by the observer.
Off State Energy
On State Energy
“ On” State (+12 deg.)
F/2.4 = 24º
Figure 1. Simplified Optical Function of a 12-Degree-Mirror-Tilt Device
Flat state (zero) occurs when the mirrors are not energized. This is not an active state of the DMD mirrors (not tristable). The resting position of the mirrors nominally is zero degrees, but the mirrors are not controlled or actuated to this position and may vary from it slightly. Flat-state mirrors exist only when the device is turned off, or parked, and no image is being formed. This application report deals only with image effects and interactions between the device and the optical system. Therefore, it is more useful to think of flat state as the integrated energy falling in the area between on- and off-state pupils during transitions of the active mirror states, plus any fixed flat-surface contributions from the device package such as window reflectance, border metal, window-aperture reflectance, lenses or prisms, etc.
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