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DLP® Discovery System Optics Application Note - page 12 / 38





12 / 38

2510332 - February 2009

  • Vertical offset requirements increase as f/No. decreases (numerical aperture increases) in order to physically separate illumination and projection optics. This is because the bundles get larger with smaller f/No. The amount of vertical offset generally determines the package height of the projector, since it must be located opposite the illumination input. In contrast to telecentric prism designs, the projection lens cannot be offset toward the illumination to minimize package height (see Figure 6). However, this characteristic can be taken advantage of in a “tower” style layout, where the projector is arranged vertically.

  • Projection lens elements on the screen side of the stop tend to become larger than telecentric elements because more of them are located on one side of the stop. Near the front (screen side) of the lens, much of the glass is not used, but truncating the glass to save weight generally is more expensive than practical, especially since it does not reduce packaging height.

  • The higher illumination angles distort the image of the integrator rod more severely at the device, which creates more overfill losses. This can be as much as 10% less efficient than a telecentric design, depending on uniformity requirements and the number and type of illumination elements used. Likewise, these higher angles tend to distort the exit pupil of the illumination system, making it difficult to define for the projection lens design and producing further losses.

  • Matching pupils at a finite distance from the device requires knowledge of the illumination system in order to design a proper projection lens, and vice-versa. This interdependence can hamper parallel-path development and increase time to market, especially if separate suppliers are involved.

  • The high offset angle produces projection angles that generally exceed current screen technology for rear-projection applications. Reducing offset is not an option, nor is variable offset, for nontelecentric designs.

  • Higher illumination angles require more clearance for the window aperture opening so that rays can enter the active area without vignetting or shadowing the active array. This requires more silicon border, or light shield, area around the active array to push bond wires and other structural artifacts out of view under the aperture. This reduces the number of DMD die that can be produced per wafer, impacting the DMD cost.

  • More off-state light is trapped in the device by the device window aperture, which can

produce undesirable thermal effects and border artifacts.

  • Magnification changes slightly with focus of the projection lens.

  • Higher offset requirements result in larger field size requirements for the projection lens. Field size is by far the single most influential design parameter for lens cost and performance. Offset cannot be optimized to minimize field size as for a telecentric prism- based design.

  • It is very difficult and expensive to design a constant f/# zoom lens for nontelecentric architectures due to change in stop position. Fixed-stop-position zoom lenses are complex and very difficult to make. Large zoom ratios tend to produce large brightness variation, accordingly. Movement of the rear group in a zoom lens also is hindered by potential interference with illumination elements, which can make large zoom ratios very difficult.


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