2510332 - February 2009
There are some advantages to using the rod or tunnel integrator, even for an LED projector:
LED’s are typically square, and DMD’s are typically rectangular. This causes mismatching of etendue and results in efficiency losses. This can be remedied by using a taper in one dimension of the integrator, with corresponding collection angles and/or anamorphic f/#’s, to “stretch” the etendue matchup efficiently if constrained to square LED die.
Color uniformity of LED systems is more difficult to achieve than spatial uniformity. Using a rod or tunnel integrator can improve color uniformity and provide consistent results.
If the LED’s were directly imaged to the DMD without an integrator, then each color would have to be aligned very accurately to the DMD to achieve good color uniformity, spatial uniformity, and efficiency. This would be much like converging a 3-panel LCD system, which is a difficult process. By combining the three (or more) LED’s into a single integrator, only the integrator need be aligned to the DMD to do this. This is similar to the way the integrator is aligned in a lamp system, adding familiarity and simplicity to the process.
There are some occasions when a fly-eye lens array integrator would be a better choice:
The fly-eye lens array eliminates the condenser lens, simplifies the relay lenses, and considerably shortens the path required for integration. It can result in a much shorter optical path, although it has some limitations in performance. For applications where minimal size is required, this is a better choice even at the expense of some efficiency.
The LED output is collected and collimated in order to pass the dichroic filters without much spreading of the bandwidth due to variation in angle of incidence on the coating. The fly-eye lens array works best with well-collimated light, so it is a good fit if collimation is done well. An example layout of a lens array system is shown below.
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