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2510332 - February 2009

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Figure 7. Spatial Irradiance Distribution of a Small-Arc Lamp at Focus of Elliptical

Reflector Before Integration (left, at rod input) and After Integration (right, at rod output) (Highly peaked Gaussian profile maps directly to screen uniformity unless redistributed by spatial integration.)

3.1.5 Relay/Folding Optics

Relay optics can be reflective, refractive, or a combination of the two. In applications using a rod or tunnel integrator (nearly all single-panel DLP applications), the relay is a classical Abbe configuration, forming an image of the rod/tunnel face at the device plane. Also, this creates a convenient field stop at the device, minimizing thermal problems and border artifacts due to illumination overfill. The function of the relay is to transfer as much of the light from the output of the integrator to the device with acceptable uniformity, and to match the numerical aperture at the integrator to the numerical aperture of the projection with appropriate magnification. Whether the relay is telecentric or not at the device, it always should be telecentric at the integrator to avoid color and spatial uniformity problems. Use of folding mirrors and the overall path length usually are determined by packaging constraints or goals, and vary from product to product. One possible benefit of curved reflective elements in the illumination relay is that these can perform the functions of both a lens and a fold mirror in one compact element. However, large off-axis angles often required for folding can have detrimental effects on uniformity, distortion, spectral transmission, polarization, and other aberrations of a curved mirror. Irradiance profile at the device always should be modeled by suitable software programs. Other considerations are:

  • Illumination relays must be optimized for minimal optical blur at the outer edges of the integrator rod or tunnel field. This is contrary to a typical imaging system, where performance usually is optimized near the center of the field. Proper weighting of the field during design optimization maximizes brightness by minimizing blur at the edges of the image, which reduces the size of the integrator cross section and, thus, the amount of overfill.

  • Integrator sizing also must account for Scheimpflug distortion caused by the angle of incidence of the illumination relay to the device. Decentering, tilting, aspherics, Scheimpflug correction of the exit face of the integrator, or some combination of these can be used to improve this in some cases. Minimizing this distortion is important for efficiency and thermal reasons.

May not be reproduced without permission from Texas Instruments Copyright 2009 Texas Instruments Incorporated

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