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DLP® Discovery System Optics Application Note - page 15 / 38





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2510332 - February 2009

  • Efficacy at small etendue. Like most projection display technologies, device panel size (area) combined with the maximum allowable numerical aperture (solid angle) determines the system etendue. This typically requires small plasma-arc sources, volumetrically constrained under high pressure. Matching the lamp etendue to the system etendue is the goal for maximum efficiency.

  • Most lamps have requirements for operating position relative to gravity. Make sure the projector layout does not violate lamp-orientation requirements in all end-use applications.

3.1.2 Reflector

The lamp reflector collects the light from the lamp and directs it into the illumination optics. Characteristics of the reflector are:

  • Cold-mirror dichroic coating to minimize downstream UV and IR loads on optical components and the DMD.

  • Elliptical (or similar) shape. Most single-panel DLP systems utilize an elliptical reflector to focus the light into a small spot on the color wheel. Systems that use parabolic shapes must add condenser optics between the lamp and color wheel to focus the lamp. As the lamp arcs approach a point source, the pure ellipse becomes more difficult to improve upon. However, lamp bulb walls may have thickness and shape variations that can cause distortions that can be corrected by higher order reflector curves.

  • It is the function of the reflector to minimize the spot size at the wheel such that the transition spokes between colors of the wheel can traverse the extent of the spot in the shortest amount of time possible. Although the mixed-color light in the spoke transitions eventually is combined into useable light, the angle subtended by the spoke transitions becomes a larger percentage of the total 360 degrees as the radius of the wheel is reduced. This leaves less time for each of the pure primary colors, eventually approaching a limit determined by the amount of time required to fit all the bits required into each particular color. This time also is a function of the lamp spectral balance, so it is a system-level problem to optimize. The TI color-wheel modeling tool should be applied to this problem.

  • The reflector must contain bulb rupture. Also, some means of protecting the color wheel from damage due to bulb rupture may be a part of the reflector assembly, such as a cover glass at the reflector exit. A cover glass is also a good place for UV and IR filters, if needed.

  • The reflector volume usually determines the thermal environment for the lamp, and, therefore, has a great impact on lamp life in the projector. Some provisions for cooling the lamp burner may be required as the reflector volume is reduced for small products.

  • Reflector surface quality becomes increasingly important as reflector surface area is reduced for small reflectors. Surface imperfections usually are a function of the manufacturing process, and become a relatively larger percentage of the total area as the reflector area is reduced for small products.

  • Avoid placing optical elements, such as lenses or windows, between the reflector face and the reflector focus. If necessary, place them as close to the reflector face or the focus as possible, and/or tilt them so that the reflected energy does not go back along a path that will focus on the lamp burner electrode tips or wires. Elements near the midway point between

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