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DLP® Discovery System Optics Application Note - page 16 / 38





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2510332 - February 2009

the face and the focus reflect lamp energy back that can focus on or near the tip of the lamp burner, unless tilted. This can cause accelerated lamp failures due to electrode oxidization or thermal gradients. For example, this can occur if a condenser lens is added to slightly change the numerical aperture of an off-the-shelf reflector rather than designing and tooling a custom reflector.

  • Be aware that the color wheel reflects a load back into the lamp/reflector assembly equal to the complement of the wheel efficiency, which can be as much as 2/3 of the lamp output. This definitely affects the thermal environment in the lamp/reflector assembly, and must be accounted for in the design.

  • The reflector curve has a large impact on the shape of the far-field angular distribution of the lamp/reflector output. This angular-weighted distribution affects the design of the color-wheel filters, and should be accounted for in pupil-weighted MTF calculations for projection-lens performance.

3.1.3 Color Wheel

The design of a color wheel is covered in a separate application note. Optically, it is a series of dichroic filters arranged in segments around the diameter of the wheel, which pass red, blue, green, yellow, magenta, cyan, or white light as the DMD device sequence requires. Some optical considerations are:

  • Dichroic filter performance as a function of angle of incidence. The smaller the spot on the color wheel, the better for timing purposes. However, usually this is achieved by increasing the speed, or numerical aperture, of the lamp reflector. This creates increasingly higher angles of incidence on the filters, changing performance and softening cutoff slopes in the process. The cost of improving cutoff performance with more coating layers should be weighed against the spot-size savings when considering going faster than about f/1 at the color wheel.

  • Location of the wheel. In all the system configurations shown, the color wheel is placed prior to the integration rod, immediately after the lamp. In field-sequential applications using a color wheel, it is more efficient to transition the color-wheel spokes through the lamp spot rather than through the integrated output of the integrator rod. This is because the lamp spot usually is much more spatially compact than the output of the rod; otherwise, there would not be a need for the rod. Also, the spoke light-recapture algorithm, if used, works better if the spoke transition is spatially mixed before reaching the device.

  • The wheel should be located as close as is practical to the integrator entrance face, considering wheel runout, vibration/shock loads, and positional tolerances. Typically, 1-mm spacing is adequate. This small space has little effect on spot size at the wheel. Typically, the focus beam waist of the lamp in this Z-axis is much longer than this space.

  • Placing the wheel prior to the integration rod also relieves significant thermal load on the rod. One thermal benefit of the wheel is that the heat from the focused spot is distributed in an annular ring of much larger surface area due to the rotation of the wheel, and there is some forced-convection cooling due to the rotation as well. However, this benefit shrinks as the wheel diameter decreases, so thermal loading of the color-wheel motor should be monitored in system design.


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