2510332 - February 2009
System Performance Tuning Tips and Techniques
Many system performance parameters have limits set by, or are influenced by, the DMD device itself. For example, if the device is replaced by a flat mirror in a typical high-quality optical system, the system contrast ratio would be at least an order of magnitude higher than with the device in place. Therefore, system performance parameters are very sensitive to how the design is optimized relative to interactions between the device and system optics, and can be optimized to achieve product differentiation and optimal performance for given applications.
For single-panel optical systems, the DMD devices usually are the limiting contributor to the full-on to full-off (FO:FO) system contrast ratio. This is the ratio of lumens projected with the device turned on (full-white screen) to the lumens projected when the device is off (full-black screen). The device alone cannot be described as having any contrast ratio, because the light exiting the device is constant, regardless of the active state of the mirrors. It is only until system pupils are defined, which constrains a solid angle of collection, that contrast can be defined because contrast can have meaning only as a system parameter. However, the device determines the limit of FO:FO system contrast ratio, so it is important to know how the device interacts with the system to affect this (and other) parameters.
ANSI checkerboard contrast is measured by projecting a checkerboard pattern of white and black squares arranged such that 50% of the area of the screen is white and 50% is black in total. In this case, light is directed through the projection lens optics; therefore, the quality of the lens design, materials, and coating processes contribute to the contrast limit.
For current production devices used in single-panel systems, the most significant factors influencing system contrast ratio are: illumination angle, mirror gap (related to mirror tilt angle), numerical aperture, and optical design/coating quality.
4.1.1 Illumination Angle
Illumination angle refers to the angle of the chief ray of the bundle incident on each device mirror. For telecentric architectures, these rays essentially are the same angle across the entire array. For nontelecentric architectures, these rays vary for every mirror across the ray due to the convergence of the illumination bundle to a finite pupil.
The illumination angle interacts with device and system optical characteristics to produce contrast-limiting conditions in several ways:
The angle determines whether the reflected flat-state light misses the projection-lens pupil, and by what margin. It also determines the location of the pupil in the off-state and the on-state, in combination with the device mirror-tilt angle.
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