DLPA052A November   2014  – August 2025 DLP9000 , DLP9000X , DLP9500 , DLPC900 , DLPC910

 

  1.   1
  2.   System Design Considerations Using TI DLP Technology down to 400 nm
  3.   Trademarks
  4. 1Introduction
  5. 2Thermal Considerations
  6. 3Duty Cycle Considerations
  7. 4Coherency Considerations
  8. 5Optical Considerations
  9. 6High De-magnification System Considerations
    1. 6.1 Incoherent Sources (Lamps and LEDs)
    2. 6.2 Coherent Sources (Lasers)
  10. 7Summary
  11. 8References
  12. 9Revision History

4 Coherency Considerations

When a DMD is illuminated with coherent, collimated, narrow-band light the reflected result is a two dimensional pattern of spots called diffraction orders. A blaze or an anti-blaze condition can exist depending on the pixel pitch, DMD micromirror tilt angle, illumination wavelength, and the incident angle of the illumination light.

A blaze condition exists when one diffraction order contains most of the energy in the overall diffraction pattern. Modeling indicates that this order can contain nearly three-quarters of the output energy, with the remaining quarter being distributed into all of the other orders.

An anti-blaze condition exists when the four brightest orders contain equal amount of energy in the diffraction pattern. Modeling indicates that these four adjacent orders can each contain roughly a sixth of the output energy (approximately two-thirds in total), with the remaining third being distributed in all of the other orders.

Basic DMD diffraction is discussed in more detail in the white paper Using Lasers with DLP® DMD Technology.

The maximum specified tilt variation between individual micromirrors is ± 1°. Near 400nm this tilt angle difference is such that customers can receive a DMD that results in any condition from anti-blazed to blazed. Therefore, the system output optics needs to have sufficient aperture to collect, at the very least, the four brightest orders in an anti-blaze condition. For example, at 405nm, a 7.56µm pitch device requires an angular aperture at least 4.4° in diameter. By increasing the diameter to 6.2°, four to five orders are captured, which is recommended.

An illumination adjustment mechanism is further recommended allowing adjustment of ± 2° from the nominal incident angle be employed in a system design. Typically, the illumination cone is centered on an angle that is 24° from the window normal so that the output cone is centered on the DMD normal for 12° tilt angle devices. The ± 2° adjustment allows the brightest orders to be moved into the output aperture in wavelengths near 405nm