2510332 - February 2009
Non-polarized light from LED’s is very efficient with DLP technology, which does not require polarization unlike LCD or LCoS technology. LED’s have negligible coherence for projector use, resulting in negligible speckle issues.
LED’s are switched on and off according to the color sequence desired. They are not continually on like an arc lamp, so no energy is wasted creating light or heat in the colors not being displayed at a given instant.
LED’s can be completely shut off when “displaying” a black screen. This results in
theoretically infinite contrast ratio.
Of course, there are also some challenges:
The primary issue with using LED’s is coupling their output efficiently into small etendue systems, like DLP or any microdisplay. Although the LED’s themselves are usually small, they emit into a very large angle, typically with a lambertian distribution. This increases the etendue of the LED and makes it very difficult to use many LED’s for a given system. Having a small LED and collecting a large angle from it is efficient, but will not necessarily produce the desired system luminance due to current density limitations of the small die. Alternately, one could collect a smaller angle and use a larger die, thus increasing the input power but reducing the collected output, which may yield higher projector luminance but at a lower efficiency. You can see from the LED collection curve in the figure below that “cutting off the tails” of the lambertian output may allow a smaller collection angle from a larger die without a big penalty in efficiency. There are a many trades to make when using LED’s.
LED Lumens Collected into Solid Angle
40 50 Cone Angle
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