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DLP® Discovery System Optics Application Note - page 20 / 38





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2510332 - February 2009

  • Typically, distortion overwhelms blur for nontelecentric illumination relays due to the much higher illumination angles to the device created by the additional offset angle.

  • Telecentric relays have lower illumination angles, and thus lower distortion, but may require more elements to control blur better because distortion is lower.

  • One benefit of field-lens architectures is that usually there are more optical elements (surfaces) in the illumination path to help optimize blur (usually a chromatic aberration). The field lens itself is decentered and tilted with respect to the illumination optical axis, which may help correct Scheimpflug distortion.

  • Vignetting can be applied to reduce the diameters of the elements required because the illumination is centered on the device. Uniformity goals should not be compromised, and brightness at the ANSI measuring points for lumens should be maintained. Keep in mind, however, that ray bundles are reversed by reflecting off the device, turning inside rays out and vice-versa. Corresponding vignetting in the projection path may be required to achieve the desired effect, or judicious placement of apertures in the illumination.

    • Always include the device window and window aperture in the model or design to make sure no shadowing of the array occurs from the window aperture, and to estimate thermal load on the window aperture due to overfill.

      • 3.1.6

        TIR Prism

Some telecentric architectures utilize a prism containing a TIR surface to separate the illumination and the projection paths in minimal space. Some design considerations are:

  • Bias the frustrated-TIR zone to the illumination side by choosing the prism angles for maximum contrast. There is an area of frustrated TIR and resonance with the AR coatings near the critical angle that prevents instant switching from TIR to refraction at the critical angle (see Figure 8). For better system contrast, it is usually better to let this failure occur in the illumination path rather than the projection path.

  • The AR coatings on the air-gap surfaces have high angles of incidence and require special attention to coating design. A reference coating design is available from TI.

  • Reflections of flat- and off-state light from the device should be managed and prevented from entering the projection lens. This can be done with the shape of the prism, absorptive coatings on nonoptical surfaces of the prism (beware thermal implications), apertures in the projection path, or some combination of these techniques.

  • Judicious vignetting can be used to minimize the size of the prism.

  • The prism air gap should be about 10 microns to prevent astigmatism in the projection path. This does not apply to RTIR designs because the air gap (if applicable) exists in the illumination path, not projection, and is of little consequence to the illumination.

  • All optical surfaces should be AR coated to minimize contrast degradation and maximize throughput. Because light goes into and out of the prism twice (double-pass), and because of the difficulty of having AR coatings in the air gap, typical overall transmission for a prism is about 92% to 93%. However, this can increase as the f/No. decreases.


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