2510332 - February 2009
4.1.3 Numerical Aperture
Numerical aperture can be used to improve contrast by certain ways of mismatching illumination- and projection-system numerical apertures. For example, if the projection-system numerical aperture is stopped down slower than the illumination system, this gives more spacing between the on-state pupil and flat- or off-state pupils, and can increase contrast. However, this comes at the expense of lumens.
Another option is to apply vignetting to the illumination system such that the corners of the device are illuminated at a lower numerical aperture (slower speed) than the center. This creates a smaller bundle for the illumination in the pupil, having the same effect as above. However, this can be done without significantly affecting brightness of the ANSI lumens measuring points, thus not decreasing ANSI lumen ratings of the system. Also, because the illumination rays are inverted/reverted by the mirror array, vignetting must occur on all the rays to those pixels. This requires two apertures in the illumination, on either side of a pupil, so rays are clipped, which defines inner and outer rays at the device. A separate application report (and patent) is available to describe this in detail.
The most effective method, however, does not change the numerical apertures overall, but selectively blocks certain areas of the pupil with a shaped aperture stop. For example, a D-shaped stop could be placed in the illumination pupil in such a way as to map to the flat-state area that is closest to the on-state (projection) pupil. This will increase contrast with only slight effect on lumens, much like as in Figure 11, even for nontelecentric systems.
4.1.4 Optical Design and Coating Quality
AR coatings for lenses or flat elements in the optical paths can affect contrast significantly, especially for telecentric architecture using a prism or a field lens. There are many paths for the reflected light from these surfaces to get through the projection lens and onto the screen, degrading contrast.
Be aware of all first-order reflected light paths from all surfaces. For illumination paths, these reflections can enter the projection path, regardless of the state of the device since they occur prior to the device. For projection paths, minimize ghost images back to the device plane, which can be reflected to the screen off the device window or other flat areas. Also, be aware of any color-filter effects from AR coatings.
Be aware of reflected light paths for the off- and flat-state light from the device, as well as flat-state light from the device window. Ensure that there are no simple paths to the projection pupil. Many optical-analysis software packages are useful for this modeling.
Elements that are between the projection lens stop and the device (including prisms and windows) have the greatest impact to contrast and should have the best affordable AR coatings and surface quality. Minimizing the number of lens elements in the projection lens between the projection lens stop and the device also is good design practice for maximum contrast. Glare stops or baffling in the lens barrel between the stop and the device also can prevent flat- and off-state light scattered or reflected into the lens from getting to the screen.
Projection lens AR coatings generally will set the limit for the ANSI checkerboard contrast, because the lens contributes scattering and veiling glare when light is passing through it
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