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

6.1 Incoherent Sources (Lamps and LEDs)

For broadband and LED sources that match the size of the illumination bundle (cone of light) to the DMD output bundle, the micromirror tilt variations can allow some light to spill off of the side or not completely fill the output aperture as shown in Figure 6-3. This results in undesired loss of output brightness.

Note:

Note that when a DMD is used with incoherent sources a filter which nearly extinguishes all wavelengths below 400 nm needs to be used in the illumination path to the DMD (see the individual DMD data sheet specification). Some LEDs do not have significant spectral content below 400 nm obviating the need for a filter.

Small Output Aperture Figure 6-3 Small Output Aperture

The best way to capture all of the light with tolerance for micromirror tilt variation is to make the illumination bundle smaller than the output aperture. This allows all of the light to be captured as illustrated in Figure 6-4:

Reflected Illumination Movement with Tilt Variation Figure 6-4 Reflected Illumination Movement with Tilt Variation

The tilt variation specification is ± 1°. At the output aperture, the reflected illumination moves 2x this amount, ± 2°, since the reflected rays move 2x the angular movement of the reflecting surface. The output aperture is recommended to be 4° larger in diameter than the illumination bundle to encompass this range (- 2° to + 2°).

Therefore, an effective limit exists on the largest ƒ number (smallest aperture) at the output that can be achieved to provide 4° of tolerance. Even if the angular extent numerical aperture (NA) of the illumination is very small, the aperture has an ƒ/14.3 equivalent which is a cone with an angular diameter of 4°.

A practical limit exits for the de-magnification that can be reached within this tolerance. Optics with an ƒ number less than one are very difficult to build. If a limit of ƒ/1 is used then a de-magnification of 13x is the largest de-magnification. The graph in Figure 6-5 shows two curves. The magenta curve is the angular diameter of the cone at the DMD output aperture that results in an ƒ/1 cone at the fabrication surface. The green curve is the allowable angular diameter of the illumination bundle that maintains a 4° margin between the illumination bundle and the output aperture. Note that the allowable illumination cone diameter reaches zero just past 13x de-magnification.

DMD Output Aperture Diameter vs De-magnification Figure 6-5 DMD Output Aperture Diameter vs De-magnification

The maximum achievable de-magnification for a given ƒ number is approximately given by:




Where θinput is the angular extent of the input illumination bundle.

In summary, incoherent sources have two limits when used in high de-magnification systems. The output is recommended to have an ƒ number less than f/14.3 and a de-magnification of 13x or less. In practice, the illumination bundle angular diameter needs to be kept at several degrees, allowing either some aperture margin to be recovered or a lower de-magnification to be chosen.