Feature: page 2 of 3
Advances in Contrast Enhancement for DLP Projection
D. Scott Dewald, D.J. Segler, and Steven M. Penn


3. Optical methods for contrast improvement

3.1 Theory of operation of the DMD

The DMD modulates input light by changing the angle of reflected light on a pixel by pixel basis. Input light is focused onto the DMD with a certain numerical aperture or f/#. For a typical design, maximum NA or minimum f/# is determined by the tilt angle of the mirrors, either 10 or 12 degrees. Light reflected from “on” pixels is reflected generally normal to the DMD plane toward the projection lens, whereas “off” state light is reflected at a higher angle (40 degrees for a 10 degree device) toward a light dump. There also exists a “flat” state, which corresponds to reflection from the window surfaces, and the mirrors themselves when they are flat. Figure 3 shows a schematic of the light cones. [3]



3.2 Diffraction and scattering effects on contrast

Understanding the mechanisms that limit contrast in DLP systems requires visualizing the light that enters the projection lens pupil in both the on state and the off state. This can be done by imaging the projection lens aperture stop onto a screen. For a telecentric illumination/projection system, the image of the projection lens stop is a mapping of the angular content of the light leaving the DMD. Figure 4 shows such an image for a telecentric DLP system using a 10 degree device at f/3.0. Note the image of the lamp reflector and the periodic structure caused by multiple images of the rod integrator entrance. Also note that the image of the lamp is not centered in the pupil. This is because the center of the illumination bundle is incident on the DMD at 22 degrees, or 2 degrees more than twice the mirror tilt angle. Also, because the DMD hinge axis is orientated 45 degrees to the row and column axes, this displacement of the lamp image is up and to the right along this 45 degree angle.



Figure 5 shows the same situation with the DMD pixels off. Note the concentration of light in the upper right corner. This corner represents angles that are very close to the flat state. The authors have identified several sources of this light, including:

1. Diffraction from the mirror active area
2. Light diffracted or scattered from gaps between mirrors and the vias.
3. Wide-angle scattering due to micro-roughness of the Al surface of the DMD mirror.

Diffracted light is contained in orders with maxima corresponding to integral values of m below [4]:

sin(è )-sin(è o) = m /d

where ë is the light wavelength, d is the period of the periodic structure, and è is the angle of the diffracted light relative to the zero order, that is, the undiffracted light corresponding to the flat state. For a 17 micron pitch DMD, the value for è is approximately 1.8 degrees. Referring to Figure 5, if the diameter of the stop represents an f/3 cone of light, then the radius of the stop represents 9.5 degrees of angle, and the diffracted orders appear to be slightly less than 2 degrees apart in the horizontal and vertical directions.



Because the light source is incoherent and spread over a large continuous range of angles and wavelengths, the diffraction orders blend together to form an somewhat uniform illumination of the pupil at angles far from the 0 order. This can be seen in Figure 5 as a semi-uniform light level in the lower-left areas of the pupil.

Scattering of light trapped between mirrors remains difficult to both measure and model accurately. While it is difficult to measure the contribution of directional scattering in angular space using images of the pupil, locations from which light is scattered can be seen in magnified images of the off pixel which will be described in the next section.

Wide-angle scattering is theorized to be caused by the micro-roughness
of the Al alloy that makes up the DMD mirror, which is deposited using a physical evaporation process. At this time, the effect of wide-angle scattering is thought to be negligible compared to scattering and diffraction effects.

3.3 Evaluation of the off-state pixel structure

To determine which locations of the pixel structure contribute to lowered contrast, the authors imaged off- state pixels onto a CCD at high magnification. A false color image of such is found in Figure 6. Arrows indicate the locations of the pixel corners and the via. It is interesting to note that the via and the corner of the pixel are significant contributors to the black level. Therefore, due to the illumination’s 45-degree angular component, diffraction and scattered light from the pixel edges seem to be negligible compared that from other places. The via, though about 2x2 microns in size, causes a significant diffraction energy even though it covers a fraction of the area of the gap between pixels. This data can be used to modify the structure of future DMD pixels for increased contrast.




Source: The Society for Information Display
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