DMD Characterization for Digital Cinema

1. DMD Characterization for Digital Cinema John Roberts Tracy Comstock NIST

2. DMD Projection Systems • High resolution • Many colors and brightness levels (grayscales) - improves realism • Fast response time, high frame rate • Good for Digital Cinema, however... • As with all displays, knowledge of display operation is needed for optimum performance!

3. Optimizing Display System Performance • Make best use of grayscale/color generation methods used by the display • Avoid “pathological cases” that degrade image quality • Evaluate a model for given classes of application - interaction of input signals, internal control algorithm • Check a specific display for correct operation

4. Basic DMD Operation Micromirrors machined into a megapixel array Light is reflected through projection optics, or into a light trap Switching time tens of microseconds Pixels are binary (fully on or fully off)

5. Generation of Colors/Grayscales • Pixels are always “on” or “off” - no inherent grayscales • Grayscales are generated by temporal modulation and spatial modulation • Colors (red, green, blue) shown sequentially, or using multiple DMDs

6. Temporal Modulation • Fast mirror switching permits many brightness levels • Binary coded pulse widths for switching control within frame • “Bit splitting” - rearrange sequence of binary time steps to reduce visible artifacts such as flashes

7. Spatial Modulation • Patterns of pixels produce variations in visible grayscale • Effective resolution is reduced • Used with temporal modulation for more grayscales, fewer visible artifacts

8. The Need for DMD Characterization • Operational details not always available to the customer • Manufacturer may not be aware of detailed needs for a specific application • Diagnosis when problems arise

9. Method of Observation • High speed screen image capture • Continuous, or periodic • Triggered, or free-running • Selected test images (animations)

10. Experimental Setup • Test images with known properties • Repeated image capture, timing offset wrt frame rate • Reconstructed animation shows mirror timing

11. Image Examples TEST PATTERN OBSERVED IMAGE IMAGE TIMING: TWO CONSECUTIVE IMAGES COMBINED RESULT

12. Designing Test Images • Horizontal, vertical gradients to look for potential critical grayscales • Blocks with known grayscales to observe spatial, temporal modulation • Add visual tags to assist optical triggering (e.g. full-red block appears in red field only) • Video tests: either rapid sequential capture camera, or short-cycle repeating animations

13. Pathological Cases • Temporal • Flicker observed at certain gray levels • Color breakup, geometric distortions • Possible workaround: remap some colors, use spatial modulation • Spatial • Regular patterns (in graphics, halftoning) interfering with modulation pattern • Workarounds: avoid deliberate use, filter

14. Application for Digital Cinema • Content creators: Test material for suitability with selected displays • Theater owners: portable device and test suite for checking installed projectors • Possible future development: extended video sequences, with mathematical analysis of captured images

15. Summary • Selected test patterns and high-speed image capture can be used to observe DMD operation and detect problems • DMD characterization can be useful for digital cinema, both in production and in testing installed systems

16. Acknowledgements • Xiao Tang, Victor McCrary, other ITL/NIST management • Charles Fenimore - Digital Cinema • Edward Kelley - display characterization • Richard Gale and Peter van Kessel, Texas Instruments