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DLP® Discovery System Optics Application Note - page 22 / 38





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2510332 - February 2009

  • Numerical aperture or f/#. Typically, this is determined by the device mirror-tilt angle to prevent overlapping flat- and on-state bundles. It limits the throughput, or etendue, for the entire projection system. However, the mirrors only steer the light along one axis. In the axis orthogonal to the steering plane, there is no functional limit to numerical aperture. In practice it is difficult to create nonsymmetrical numerical apertures, but the benefit can be large.

  • Focus range. This is the range of distances from the screen to the projector within which the image is expected to be in focus. Although not difficult for a fixed-focal-length lens to accommodate, it has a significant effect on the design of zoom mechanisms. Also, it is important to consider the tolerance for the location of the device plane due to variations in die height and/or package type when designing focus mechanisms, particularly zoom lenses.

Image distortion. Typically, a design goal of ±1% maximum distortion is required for acceptable performance with projectors used for graphics. This can be 2% total distortion if there are no inflections. Even higher amounts are acceptable for video or photo type applications.

Lateral color. A single-panel DLP system is permanently converged, by definition, as opposed to a three-panel LCD system in which each panel must be made to align with the other two on the screen. Over time, a the three-panel LCD mechanism drifts out of alignment, creating secondary color artifacts around the pixels. However, lateral color aberration in a projection lens can produce pixel color artifacts that appear similar to misconvergence of three-panel systems. For most graphics applications, lateral color of less than 1 pixel from 430 to 670 nm gives acceptable performance. Experience has indicated that the MTF requirements defined below usually can be met with ease if the lateral color requirement is met. Field size is a very strong factor in lateral color correction.

Field size. The device active-area dimensions and the amount of offset required for keystone correction determine the size of the field that the projection lens must image to the screen. In general, field size is by far the strongest factor determining the lens complexity, size, cost, and performance limits. Any relief in field size requirements usually yields big dividends. Because performance goals and panel sizes are fixed, offset is the key variable to scrutinize. For a nontelecentric system, or certain field lens systems, offset is not selectable. In those cases, offset is required to physically separate the illumination and projection optics or to control ghost surface reflection paths. However, for a telecentric system using a TIR prism, offset can be any amount desired, including zero. In section 4 of this application report, more detail is given about minimizing offset, while achieving acceptable system keystone performance.

MTF. Graphics projection is a more demanding application than video for image quality because graphics map directly to pixels and typically consist of many lines and characters that are orthogonal to one another. MTF is the metric for describing how well a lens resolves, or focuses, an image feature. It is specified in two orthogonal directions, sagittal and tangential. Please refer to any standard optical textbook for details about MTF and how it is measured, if needed. TI recommends optimizing MTF in the design of the lens by photopically weighting the spectrum and by angular weighting of the pupil according to the lamp/reflector far-field distribution, in order to achieve the best correlation to actual projector performance. TI recommends a minimum of 40% MTF (average of sagittal and tangential)


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