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Advances in Contrast Enhancement for DLP Projection
D. Scott Dewald, D.J. Segler, and Steven M. Penn


3.4 Control of diffracted light by pupil shaping

Use of a shaped projection lens aperture to improve contrast has been proposed [5], but TI has found by experiment that, ideally, both the illumination ray bundle and the projection lens must have shaped pertures. In addition, it was found that the optimal shape and orientation of the illumination and projection apertures is somewhat different than that proposed in [5].

In general any blocking of the projection lens pupil will result in a reduction in efficiency, but this can be minimized by using a shift in the illumination light bundle toward a higher angle and blocking the part of the pupil that is not filled with light from the lamp. As the illumination angle is increased, the flat state, which also includes the zero-order of the diffracted light, increases in angle accordingly. This causes more angular separation of the flat state from the projection lens pupil, so fewer of the diffracted orders enter the pupil. Depending on the distribution of light in the illumination bundle, some fraction of the light reflected from the DMD in the on state is allowed to miss the projection lens pupil, causing a brightness decrease.

The projection lens pupil is now illuminated non-symmetrically (Figure 4) with a section of the pupil not illuminated at all. This region of the pupil still contributes to the black level, or off state, without contributing to the on state. Therefore this region can be blocked in the projection lens without having a large effect on brightness. After experimenting with several geometrical shapes, it was found that the “cat-eye” shape of Figure 8 resulted in the best compromise between light loss and contrast increase.



4. Experimental Results

Experiments were performed on several DLP projection systems, both prototypes and production models, to quantify the contrast increase vs. brightness decrease caused by incorporating the combination of increased illumination angle and shaped apertures in the optics.

The results of the first experiment are shown in Figure 9. The optics platform was a prototype f/3 telecentric RPTV engine using a Philips 120W UHP lamp in a parabolic reflector. The arc was focused onto a rectangular rod integrator using an aspheric condenser. The intent of this experiment was to quantify the increase in contrast relative to brightness loss as the illumination cone central angle is varied from the theoretical value of 2X the DMD tilt angle, leaving all other aspects of the optics constant. This causes a shift of the pupil illumination similar to that shown in Figure 4. There is a significant increase in contrast with a small 46.1 / D. S. Dewald
decrease in brightness as the illumination angle is increased 2-3
degrees. This effect was observed with both 10 degree and 12 degree DMD’s.



A second experiment used a combination of illumination angle increase and projection lens and illumination optics cat-eye apertures. The optics platform was a production model RPTV engine using a fly-eye (lens array) integrator and parabolic lamp. The engine was retrofitted with a 12 degree DMD. Results are shown in Figure 10. Note the dramatic increase in contrast when both the apertures are used, and that the addition of an aperture that matches the shape and orientation of the aperture used in the projection lens does not have a significant impact on light throughput.

It is clear from the experiments that the use of both illumination and projection pupil-shaping apertures is recommended for highest contrast. The goal of the projection lens pupil aperture is to control light that is diffracted and scattered into angles very close to the zero-order. Illumination light is then apertured such that all of the zero-order passes through the projection lens pupil (projection and illumination pupils have the same shape and size in angular space). This creates the optimum condition for obtaining maximum contrast improvement with lowest brightness loss.





5. Discussion

An image of the off-state pixel was taken after the contrast enhancing apertures were in place, and is shown in Figure 11. Note that the relative brightness of the via to the pixel corners Is lower when the shaped apertures are used. This implies that a majority of the light blocked by the apertures is diffracted by the via rather than the pixel edges or corners. Analysis of this image shows that approximately 50% of the off-state light is diffracted by the via with apertures, versus >70% when no aperutres are used. Clearly the via is a significant contributor to black level and its removal could lead to a significant increase in the contrast ratio.

DLP Products has embarked on a program to modify the pixel structure to take advantage of the knowledge gained from this contrast enhancement effort. Based on the above data, the contrast ratio of future projectors using DLP technology is expected to increase significantly.

6. Acknowledgements

The authors would like to thank Erik Wilson of DLP Products for his thorough experiments and concise recording and presentation of the data. They would also like to thank Terry Bartlett, Danny Pyles, and Dan Cahill for their support.

7. References

[1] Poynton, Charles A. A Technical Introduction to Digital Video, p. 85-87 (Wiley 1996).
[2] Segler, D.J., and Pettitt, Greg. The Importance of Contrast for Rear Projection Televisions and its Effect on Image
Quality. SMPTE, 36th Advanced Motion Imaging Conference (2002).
[3] Hornbeck, Larry J., A Digital Light Processing Update,
IS&T/SPIE Projection Displays V (January 1999), p. 158
[4] Born and Wolf, Principles of Optics, p. 449 (Cambridge, 1959, 1999).

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