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DLP9000: Diffraction pattern observed when imaging DMD chip

Part Number: DLP9000
Other Parts Discussed in Thread: DLPLCRC900DEVM

Hi,

I am using a red laser to illuminate the DMD with the goal of imaging the DMD surface. As shown in the figure below, there are two 4-f systems before camera.

L1 and OL1 forms the first 4f system; OL2 and L2 forms the second 4f system. L1, L2 are identical; OL1, OL2 are also identical. 

So this system is imaging DMD surface on the camera without maginification or demagnification.

 

However, the image we get has some obvious diffraction at the edge of a random pattern we load on DMD. This pattern also exist when we switch to a broadband light source. The figures below are under laser and broadband source, respectively. The size of the smallest square here corresponds to 40x40 pixels on the DMD. Do you have any idea where does this come from? In presence of this diffraction, it seems that we can not make use of every pixel on DMD for modulation.

  • Hello Cheng,

    Do you mean that the two sides have an f # of 4 on each side? 

    I also surmise that the top image is with the red laser and that the bottom is with the broadband illumination.  Is this correct?

    Could this be an interaction between the periodicity of the DMD and the periodicity of the camera sensor array?  I quick experiment to check for this would be to rotate the camera 45 degrees and take an image to see if the effect changes significantly.

    Fizix

  • Hi Fizix,

    Thanks for the prompt response.

    The 4-f system I mentioned has a detailed explanation here: https://www.opticsforhire.com/blog/4f-optical-system-fourier-optics. To be short, it just makes a conjugate plane of the DMD surface.

    What you said about the laser and broadband illumination images are correct. I quickly rotate the camera for 45 degrees but it almost makes no change to the image. Please see the image below. Do you see similar diffraction pattern before?

    Best,

    Cheng

  • Hello again Cheng,

    Thank you for the picture.  I have to agree that this does not appear to be an interaction of the spatial periods of the DMD and the camera sensor.

    Also thank you for the link to the 4 f system explanation.

    Are you using the DLPLCRC900DEVM with the DLPLCR900EVM?  

    Fizix

  • Hello again Cheng,

    I looked closely at your pictures and do not think this is diffraction.  I suspect that you are rotating the image by 45 degrees on the DMD surface so that the "straight" edges show a zigzag boundary. 

    Please try rotating the pattern on the DMD by 45 degrees.  If this works then please verify this as the solution.

    Fizix

  • Hi Fizix,

    Rotating the pattern by 45 degree solve my problem. It turns out the original pattern already has some 'abberation' at the edge. Thank you for the help!

    Best,

    Cheng