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DLP® Discovery System Optics Application Note - page 27 / 38





27 / 38

2510332 - February 2009

  • For example, in Figure 11, increasing the illumination angle to 22 degrees for this device in this system would improve FO:FO contrast by about 14%, while decreasing lumens about 4%, which may be a good tradeoff for certain applications requiring the best contrast performance, such as video applications in darkened rooms or rear-projection. In applications where real contrast more likely is limited by ambient room conditions, this improvement in contrast may not be as important as lumens. In fact, room-limited contrast for front projection actually will be improved by maximizing projector lumens at the expense of some actual projector contrast because the black level will be set by the room (not the projector), but the white level will be set by the projector (lumens).

  • Also, Figure 11 shows that increasing the illumination angle even further continues to produce contrast gains, but at the expense of lumens in a telecentric system. This is because the pupil of the illumination system increasingly is not steered back into the projection pupil by activating the device to on; it can steer only by 2X the tilt angle. One may be tempted to avoid the loss in a telecentric system by oversizing the pupil in the projection lens, but that makes the lens larger and more expensive and causes it to pick up more flat- state light, thus defeating the purpose. Another way is to decenter a smaller pupil in the projection lens, but this is very difficult and complex to implement mechanically, and does not make the lens any smaller or less expensive. If maximum contrast is required with maximum lumens, consider employing a nontelecentric architecture with offset projection. In this case, the offset angle increases the nominal illumination angle much higher (near 30 degrees or more at the center of the device), which Figure 10 shows has significant impact on system contrast because a large portion of the scattered light from the device is now avoided. However, unlike telecentric systems, the projection lens pupil is nominally located to match the illumination pupil without oversizing, since it is not located at infinity. Therefore, there are no lumen losses traded for the highest overall contrast performance in this architecture.

4.1.2 Mirror Gap

The device mirrors must have a small gap between them to allow them to rotate independently of each other in the on or off state without colliding with each other. The size of the gap is a function of the mirror size and the mirror-tilt angle. Tilt angle affects the gap because the mirrors also translate slightly as they rotate because they are supported by a post that displaces them from the hinge axis. The size of the gap determines the amount of structure under the mirrors that is exposed to illumination light, and strongly influences contrast ratio. For example, an increase in pixel gap of only 0.1 micron can produce as much as 20% contrast degradation in a telecentric architecture. Nontelecentric architectures are less sensitive to this effect due to the higher overall illumination angles.

May not be reproduced without permission from Texas Instruments Copyright 2009 Texas Instruments Incorporated


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