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DLP4500: DMD pixel switching shows high intensity 'overshoot' in pattern sequence mode

Pascal Rietz
Prodigy 40 points
Part Number: DLP4500

Tool/software:

Dear all,

I am using a DLP4500 illuminated by a 405nm laser. I have measured the intensity with a photodiode and an oscilloscope. I used the pattern sequence mode and displayed a test pattern, which shows an image of just one color. The RGB values are decomposed into 24 bit planes. When switching the entire DMD from 'on' to 'off', there is some "overshoot" visible (see two exemplary images below). This means, the measured intensity is higher for a period of ~100µs. Can you tell me what is the origin of this?

If all the pixels are in the 'on' state without this overshoot, are there some pixels which are not addressed by the normal pattern switching...?

Thank you in advance for your answer.

Best regards,

Pascal

  • 0
    TI__Guru 50360 points

    Hello Pascal,

    This is almost certainly a result of the landed tilt angle being just slightly larger than the "blaze" angle for the particular wavelength of light.  Therefore the mirrors briefly pass through the blaze angle which is gives a peak in the throughput.  Judging from the second image at 30 us/div it appears that the peak is around 5 us.  The first image is a little harder to tell but seems to be a little longer.

    Please see the following Application note:  Using Lasers With DLP DMD Technology.

    Fizix.

  • 0 in reply to Fizix
    Prodigy 40 points

    Hello Fizix,

    Thank you for your swift reply. In general, your answer seems plausible to me that this can cause some peak when switching the mirrors.

    However, in my case, there is a plateau of a higher signal that lasts for about 100 µs (see the zoomed-in oscilloscope screenshot attached). Therefore, this does not seem to be an effect that is just present while switching the mirrors. The scale in the screenshot below is 100 µs/div. In the screenshot above, the scale is 1 ms/div, not 30 µs/div.

    Could this still be explained by some mirror tilt angle movement? Why would this movement happen?

    Best regards,

    Pascal

  • 0 in reply to Pascal Rietz
    TI__Guru 50360 points

    Hello again Pascal,

    Thank you for the higher time resolution scope shot.  I have to agree this does not appear to be a transient during mirror cross-over.  Can you tell me what firmware you are using?

    90us seems to be about the load time of the array.  I will have to check.

    Is this a free space optics setup? (i.e. collimated laser no other optics)? How much of the DMD is the laser spot striking?

    I assume that after that you are collecting with some kind of photodiode.  This would be equivialent to a small aperture system.  In that case when the array switches the mirrors change angle slightly when the underlying memory cells change to the off state.

    Fizix

  • 0 in reply to Fizix
    Prodigy 40 points

    Hello Fizix,

    currently, I am using firmware version 4.4 from TI. I have seen this however also with firmware version 3.0.

    Yes, it is a free space optics setup. The DMD is illuminated by a nominally collimated circular laser beam with a beam diameter of ~5mm (DMD active area: ~6.2mm x 9.9mm). 

    For this test, after the DMD there is a single lens and a photodiode. I do agree with you that the effect I measured most probably comes from a mirror angle change.

    Is there some way to get around this mirror movement when the memory cells change to the 'off' state? Can you quantify how large this mirror movement can be? Currently, it is hard for me to quantify this experimentally, but maybe I can estimate whether this is still in specifications.

    Best,

    Pascal

  • +1 in reply to Pascal Rietz
    TI__Guru 50360 points

    Hello Again Pascal,

    Is your end application going to employ free space optics or you be using collection and focusing optics?  If so, then having a larger effective NA should mitigate this effect.  

    When the mirrors load data for pixels going from 1 to 0 (on to off) the mirrors change angle slightly.  In typical projection systems it is smoothed out by the range of angles in the f# of projection optics.  In coherent free-space optics it is much more noticeable.

    Fizix

  • 0 in reply to Fizix
    Prodigy 40 points

    Hello Fizix,

    thank you for your comment regarding the NA. I think I will have to try out if this causes any problems for my application in the end.

    Best regards,

    Pascal

  • 0 in reply to Pascal Rietz
    TI__Guru 50360 points

    Hello Pascal,

    Please let me know what you discover.

    Fizix