What causes a purple blotch that only appears at high gain/high ISO, and is it sensor damage?
Asked 7/29/2018
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2 answers
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I’m seeing a large purple, ghost-like blotch over part of the frame from a digital camera. It only shows up at night when the camera raises gain/ISO, and it disappears in daylight or when the scene is bright. Lowering the gain makes it mostly go away, but that hurts low-light visibility.
The shape stays fixed even when zooming, so it does not seem to be dust or a mark on the lens. What kind of sensor-related issue could cause a fixed purple stain like this? Is it likely to be sensor damage, or is it more consistent with a noise/non-uniformity problem? Can it be corrected other than by lowering gain or applying noise reduction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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The defect may be an example of Fixed-Pattern Noise, which worsen with high ISO, prolonged use, and increased temperatures. FPN is usually associated with long exposures, although there are different types that may occur in different situations.
DSNU (dark signal non-uniformity) is the offset from the average across the imaging array at a particular setting (temperature, integration time) but not external illumination
PRNU (photo response non-uniformity), which describes the gain or ratio between optical power on a pixel versus the electrical signal output.
You may be able to avoid the problem by using lower ISO settings or noise reduction techniques that specifically target FPN.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
8y ago
0
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This is most consistent with fixed-pattern noise (FPN) or sensor non-uniformity rather than a lens issue. Because the blotch stays in the same place as you zoom and mainly appears when gain/ISO is high, it points to the sensor/electronics becoming visible under low-light amplification.
Two related forms are often involved:
- DSNU: dark signal non-uniformity, where some parts of the sensor have a different offset in darkness
- PRNU: photo response non-uniformity, where parts of the sensor respond differently to light
These issues tend to become more obvious at high ISO/gain, with longer use, and at higher temperatures. That matches a defect that is invisible in bright conditions but appears strongly in dim scenes.
So it may not be “damage” in the sense of a burned or scratched sensor; it can simply be a sensor calibration/uniformity problem that is exposed by heavy amplification. Practical mitigation is limited to lower gain/ISO and noise reduction methods that specifically target fixed-pattern noise.
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