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"Black sun" effect in CMOS sensors

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  1. Feb 22, 2015 #1
    Hi,
    I would like to understand the reason for the "Black sun" effect that occurs in CMOS cameras (very bright spots appear as dark).

    Thanks,
    ALex
     
  2. jcsd
  3. Feb 24, 2015 #2

    Baluncore

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    I do not know, but here are a couple of possibilities.

    If the semiconductor sensor is heated it may become conductive and discharge the image faster than it can be integrated, giving zero charge = black.

    In analogue television systems, a noise pulse exceeding white was gated to produce a dark spot on the screen. Dark noise is less distracting than bright noise. Maybe the same concept is being applied to digital sensors.

    The fact that it can be removed by software suggests the software looks for the bright area and paints white in the hole.
     
  4. Feb 24, 2015 #3

    davenn

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    I haven't seen this effect on any of my cameras with CMOS sensors4 or 5 cameras .... do you have any image examples
    that also show image details ... shutter speed, f-stop ?
     
  5. Feb 26, 2015 #4
  6. Feb 26, 2015 #5

    Baluncore

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    It might be interesting, but I cannot download a 26MByte file over my slow link. Can someone please review it and post an extract of the relevant paragraph?
     
  7. Feb 26, 2015 #6
    So sorry, I didn't notice the size of the file. It says (verbatim):
    "If too much light impinges on the pixel of some CMOS image sensors, here as well the potential pot can spill over (called blooming for CCDs) and cause an increase of the reference level. This in turn causes due to the subtraction of signal and reference negative values appearing as „black spots“ in the bright areas."

    I think what the author meant to say was:
    "If too much light impinges on a given pixel in some CMOS image sensors, the potential well of that pixel can spill over and increase the reference level (this is called 'blooming' for CCD sensors). This in turn, causes an error in the sensor's output due to the subtraction of signal and reference. Since the subtraction results in a negative value, the output of the sensor is set to zero. Thus the 'black spots' appear in a field of bright pixels."
    I hope I didn't butcher the author's meaning too much.
     
    Last edited: Feb 26, 2015
  8. Feb 26, 2015 #7

    davenn

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    interesting ... yes, am well familiar with blooming on CCD sensors
    just have never see the black spot effect

    Dave
     
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