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DLP7000: DMD have petal-like shapes

Joselito Go
Expert 5810 points
Part Number: DLP7000

Hi Experts,

Good day.

Our customer is using DLP7000, and the laser is using 532nm, and they want to do single-pixel imaging.

Now they have some problems, why does the checkerboard grid reflected from the DMD have petal-like shapes
(as shown in the following figure: the left is the reflected light spot, and the right is the light path I use for the positive incidence).
Please advise how to solve this problem.



Keep safe.


Regards,

Josel

  • 0
    TI__Guru 54175 points

    Hello Josel,

    I don't know how I missed your post yesterday.  This is the Fourier transform of the illuminated pixels on the DMD.  It appears to be mid-field between a near field pattern and a far field pattern.  This is a result of an interaction between the temporal coherence of the laser and the spatial coherence of the DMD pixels and the checkerboard pattern.

    Judging from the provided image it looks like they are illuminating most of the DMD.  Is that a correct assessment?

    Please have them use a slow lens and actually re-image the surface of the DMD on the target screen.  Then they should properly see the checkerboard pattern.  

    Fizix

  • 0 in reply to Fizix
    TI__Guru 54175 points

    Josel,

    Also please ask what the size of the checkerboard and also the individual squares is in pixels? 

    Fizix

  • 0 in reply to Fizix
    TI__Expert 5810 points

    Hi Fizix,

    The customer replied:

    You judge accurately. The 532nm laser occupies up and overflows the DMD
     
    The size of the checkerboard is to occupy the entire DMD (1024x768), the pixels of each square is 128, and the single micromirror is 13.7um

    What is Slow Lens？

    Regards,

    Josel
  • 0 in reply to Joselito Go
    TI__Guru 54175 points

    Hello Josel,

    A "slow" lens is a lens with a large focal distance.  A "fast" lens has a short focal distance (more curved).  They may need someone with optical knowledge to help them set this up.

    Fizix

  • 0 in reply to Fizix
    TI__Expert 5810 points

    Hi Fizix,

    Good day.

    The customer replied:

    Why can you see the checkerboard pattern with the slow lens? How can I explain it theoretically?
    The other question is whether you choose normal incidence or 45 degrees, what does that affect?


    Regards,

    Josel
  • 0 in reply to Joselito Go
    TI__Guru 54175 points

    Josel,

    Because the added lens is itself another Fourier transform element which transforms back to the spatial domain from the spatial frequency domain.  This puts the phase information back together so that the apparent artifacts in the intermediate image are re-integrated into an image that you can recognize.

    If you do the same experiment with nearly collimated white light, the intermediate image will be nearly unrecognizable, but when the lens is added it will re-integrate back into the image of the DMD surface.

    The primary reason I suggested a long focal length lens was because of the distance over which they were attempting to obtain an image.  It is not strictly required, but easier to maintain a larger final image.

    Fizix