2510332 - February 2009
Disadvantages of Telecentric Architecture Telecentric architectures have some disadvantages/challenges relative to others:
Prism-based systems have additional costs, size, and weight of the prism. The TIR air gap
has high angles of incidence, causing some polarization effects and greatly increasing the difficulty of achieving good AR coating designs. TI has reference design coatings available to minimize development efforts for these coatings.
TIR air-gap coatings have relatively high losses, 2% to 3% per surface. These losses tend to offset the gains from reduced distortion overfill losses. However, as the f/No. decreases, these losses tend to decrease as well (coatings become more efficient).
TIR prism surfaces also can produce surface reflections that enter the projection pupil, even though these surfaces are flat. This is because the intersection of the illumination bundle with these surfaces is displaced from the projection path, creating reflected flat-state light that is displaced from the flat-state pupil location defined by the illumination optics. This light may go to the screen if it can enter the projection lens aperture and pass through the pupil, and it is not controlled by the device state. This also is true for the DMD device window- surface reflections, regardless of architecture. All flat surfaces near the device must have very effective AR coatings to minimize this effect, and their reflections should be thoroughly traced/modeled for possible contrast degradation. The shorter back working distances of telecentric projection lenses, while a benefit to size of the optics, is a detriment to contrast because of the lack of sufficient space to physically separate the on bundle from flat and off bundles.
One of the strongest factors affecting system contrast is illumination angle to the device. In general, the higher the angle, the higher the contrast (more detail in section 4.1.1). Telecentric designs have lower angles of illumination than nontelecentric designs due to lack of additional offset angle. This can reduces inherent contrast compared to nontelecentric, although it inherently is more uniform. Increasing illumination angle alone increases contrast, but also offsets the pupil in the projection lens and introduces vignetting if the numerical aperture of the projection lens is not increased accordingly. However, if the projection lens numerical aperture is increased to avoid vignetting, it can collect more flat- state and stray light from around the device and pass it to the screen, thus potentially defeating the initial intent of improving contrast. It is a tradeoff that is dependent on system requirements.
As projection offset is added for keystone correction, the elements in the rear of the projection lens prior to the stop increase in diameter proportionally with the increase in field, because the ray bundles exit the device perpendicular to it. However, selecting only the amount of offset necessary for the application can minimize this. This is not an option for nontelecentric designs.
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