2510332 - February 2009
Nontelecentric architectures differ from telecentric in that the exit pupil of the illumination path is located a short, finite distance from the device, and the entrance pupil of the projection lens must be coincident with it (see Figure 4). Since some degree of vertical projection offset usually is required for most front-screen applications, additional illumination angle is added to offset the pupil in the vertical axis for the projection lens. This adds additional angle of incidence to the device, increasing inherent contrast, while providing more angular separation of the illumination path from the projection path. This additional angle makes it difficult to use a TIR-type prism for separating the paths, but a field lens (or lenses) can be used instead. Typically, the separation is in air space for minimal cost (fewest optical elements, smallest size elements). However, a field lens in this space reduces path lengths and allows more compact use of the space in front of the DMD.
Since the bundles are converging to the pupil, the angle of incidence of the chief ray for each mirror on the device varies with position in the array. Although this can produce nonuniformity of the dark field (black level), the higher average illumination angles due to the additional offset angle tends to increase the contrast (reduces black level). Also, this convergence to the projection lens minimizes the diameter of the projection lenses on the DMD side of the stop, further enhancing physical separation of the two paths.
Some designs use a field lens (or lenses) instead of a prism directly in front of the device to perform the angle separation. The field lens must be on axis with the remainder of the projection lenses, but is shared by the illumination path. This presents some challenges in illumination design since these field lenses also are part of the illumination path, but are off-axis to the DMD and tilted to the illumination path. By designing the illumination pupil near the stop of the projection lens and folding the illumination path there, a very compact system can be designed if attention is paid to the unique challenges this design presents.
May not be reproduced without permission from Texas Instruments Copyright 2009 Texas Instruments Incorporated