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DLP® Discovery System Optics Application Note - page 37 / 38





37 / 38

2510332 - February 2009

  • Since they are essentially very small point sources and emit into relatively low angular space, the etendue of the laser source is typically very small. This would theoretically allow a very small DMD to be coupled very efficiently to a laser, thus resulting in a large increase in die per wafer and a significant cost reduction for the DMD. In practice, several things conspire against this, which will be addressed later.

  • The etendue of the source laser is much smaller than the DMD, therefore there are many more options for combining colors at the DMD that do not involve dichroic filter combination optics.

  • Small, highly collimated sources result in the ability to use very high f/# optics. This reduces the size of the optics significantly, and increases depth of focus so that focus adjustments are often eliminated, thus simplifying the mechanics as well. This should produce significant cost savings in both the optics and mechanics of a system, and significant reductions in size and volume of an optical engine.

  • Lasers typically have much narrower spectral distribution than comparable LED’s, which can be important in some applications such as biomedical, chemical, communications, etc.

There are many potential challenges for laser applications:

  • Serious regulatory and bureaucracy issues that present obstacles to getting products to market. Consumer acceptance of eye safety concerns, perceived or real.

  • Laser light is typically polarized. While there is no efficiency penalty for unpolarized DLP technology with polarized light, neither is there a competitive efficiency advantage over polarized technologies such as LCD or LCoS as there is with unpolarized LED’s.

  • The spatial coherence (due to small etendue) and temporal coherence (due to narrow wavelength spectrum) both contribute to serious speckle issues that are difficult to reduce to acceptable levels, and can require optical measures that usually expand etendue and/or bandwidth so much that the small-source etendue advantage of lasers is severely diminished.

  • Energy density levels of laser sources for high brightness projection can require optical materials and workmanship quality that prohibit cost effectiveness.

  • The manufacturing base for LED’s is very large compared to lasers because there are very large competitive markets for LED’s outside of projection applications. Lasers are typically created for specific applications. The relative economies of scale and pace of innovation are very different as a result. One possible exception may be the laser printer market as it may apply to laser sources for pico projection applications.

  • The narrow laser wavelength spectrum can produce colors that may be “oversaturated” for many display applications, but can often be desaturated by adding small amounts of the other primary laser colors. This can improve luminance by reducing the gamut, but requires higher duty cycles for the lasers which can be a thermal or power consumption issue.

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


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