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Off-pixel brightness

Anthony Zorzos
Prodigy 200 points

I am using a laser source for DMD illumination (no projection optics).  For the projected image, the off-state pixels are yielding non-zero brightness values.  I believe this may be due to the glass window over the DMD mirrors.  Is there a way to deal with this issue (perhaps controlling the incident angle, or removing the glass) ? Thank you.

  • 0
    TI__Guru 54430 points

    Hello Anthony,

    What is the wavelength of the laser you are using?  What direction are you collecting as your "projected image" and what is your incident condition (angle from the normal and angle from the top of the device)?  This is similar to "altitude and azimuth" except altitude is measured from the normal not the horizon and "North" (i.e. 0°) is the top of the array.

    You may be getting a diffraction order with a small amount of energy in it when in the "off" state.  What happens if you turn all the mirrors off"

    P.S.  Even in a regular projector, the "off" state is dark, but not zero if the illumination is left on.  "Dynamic Contrast Ratio" is misleading since the illumination is also modulated in many consumer projectors.  A better indicator for your purposes is called "ANSI Contrast Ratio".  Wikipedia has a pretty good discussion on this --> http://en.wikipedia.org/wiki/Contrast_ratio

    I hope this helps.

    Fizix.

  • 0 in reply to Fizix
    Prodigy 200 points

    Hi Fizix,

    The wavelength is 473 nm.  The incident laser beam is altitude: -45deg, azimuth: -30deg.  The projected laser beam (when all the pixels are in the on-state) is altitude: +20deg, azimuth: ~0deg.  Obviously, these are all estimates. 

    When all the mirrors are turned off, I can observe no projected image (along the on-state path).

    I have no projection optics after the DMD (I am simply using the reflected image).  It seems as though the farther from the DMD the image is, the worse the contrast is for the image (I'm simply projecting a checkerboard image).  Thanks again,

    Anthony

  • 0 in reply to Anthony Zorzos
    Prodigy 200 points

    Hi Fizix,

    Continuing my last response...

    Do you think this may be due to laser beam divergence?  At a distance, the projected image is surely larger.  Thanks,

    Anthony

  • 0 in reply to Anthony Zorzos
    TI__Guru 54430 points

    Anthony,

    Is there any possiblity of posting a few pictures of what you are seeing?  If you do not want to put them in the public forum please send me a friend invitation and start a private conversation.

    Fizix