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DLP® Discovery System Optics Application Note - page 21 / 38





21 / 38


2510332 - February 2009

Reflectance – %

“Frustrated TIR” light is trapped in the TIR prism and can reduce contrast



Angle – degrees

Figure 8. Frustrated TIR in TIR Prism Air Gap Should be Biased Toward Illumination Path

Projection-System Components and Design Parameters

The function of the projection system is to magnify an image of the device to a screen, while maintaining throughput and uniformity. It consists of a projection-lens assembly (fixed-focal length or zoom), possibly a TIR or RTIR prism, and the device. If used, a TIR prism in the projection path basically is a flat glass plate and has little effect unless the air gap is large enough to introduce astigmatism. The device window also is a flat glass plate that should be included in the design model. The performance of the system can be described and measured in classical metrics for an imaging system, such as modulation transfer function (MTF), specific image aberrations, numerical aperture, etc. As for other imaging systems, the design of a projection lens is a balance of performance, cost, size, weight, volume, environmental requirements, and other system parameters. Factors influencing projection lens design are:

  • Throw ratio. The ANSI definition of throw ratio is the distance to the screen image from the projector divided by the width of the image. There are many other definitions, such as those based on the image diagonal or the inverse of these relationships, so be sure there is mutual understanding when discussing throw ratio. Throw ratio is determined by the focal length of the projection lens. Typically, it is constrained by the application desired, but, in general, the longer the throw ratio the longer the focal length and the smaller the lens. For the smallest lens possible, make the throw ratio as long as possible for the application. Typical throw ratios for conference room or mobile front-projectors are in the range of 1.5:1 to 2.2:1. For rear-screen projection in a TV application, the throw required usually is limited by the screen technology or cabinet layout and usually is much less than 1.0:1 (typically,

    • 0.55


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