2510332 - February 2009
(unlike FO:FO contrast). Typically, if the FO:FO contrast is not much higher than the ANSI checkerboard contrast, the projection lens is performing well but there is some element in the illumination path that has a reflection getting to the screen. This typically indicates poor TIR prism coatings, or lack of ghost-reflection control on field-lens elements. If FO:FO and ANSI checkerboard both are low, the illumination has a serious problem and the projection lens cannot be evaluated until the noise floor is lowered by improving the illumination. If the FO:FO is where it should be, but ANSI checkerboard is much worse than the FO:FO, there likely are coating problems in the lens, or serious ghost-image issues. It also is important to baffle the lens barrel behind the stop (towards the device) as well as possible because it is likely to pick up flat- or even off-state light in a highly offset telecentric design as the rear of the lens grows with offset field. Nontelecentric designs avoid most of these problems, but can have more issues with control of the flat-state light reflecting off the device. It is very important to model device window reflections for a nontelecentric design if contrast and border image artifacts are important to the system design.
The general relationship between lumens and contrast has been described in the contrast discussion in section 4.1. However, there is a possibility that products using certain devices may encounter a situation where more than one tilt angle may be available for the same device kit (device and compatible ASIC and other electronics). TI can recommend an optical platform that is compatible for multiple tilt angle devices, while achieving maximum lumen performance for each. Please contact TI for this application.
Optimizing Optical Costs
This section addresses considerations that can help prevent unnecessary costs due to overspecification of the optical design of the projector system.
4.3.1 UV Filtering
The device has specifications for maximum exposure levels of ultraviolet (UV) radiation for maximum lifetime reliability. Many types of glasses, glass coatings, and/or mirror coatings used in illumination optical elements can attenuate UV naturally. It is recommended that actual UV levels be measured at the device on early prototypes to determine how much, if any, additional filtering may be required. It may be helpful to design the projector optical engine tooling to provide a convenient place to mount a low-cost plate-glass UV-reduction coated filter if needed, and then use it only if needed. UV coatings are relatively expensive, so it is best to put them on small flat surfaces to maximize parts-per-coating-run costs, if needed. Also, lower cost filters are as low as 90% to 95% transmissive in the visible range, costing many lumens (and generating high local heat loads that must be heat-sunk effectively).
If these filters are located between the lamp and the color-wheel or integrator, they should be tilted at an angle to prevent localized heating of the lamp burner electrodes or wires by the back- reflected radiation. This tilt should also be accounted for in the specification of the filter for angle of incidence.
DMD’s specifically designed for use with UV light are available for applications requiring UV illumination. Please contact TI for specifics for these applications.
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