I'm trying to understand what is causing chroma noise in an image sensor, specifically why it is so visible when the ISO is set to a very high value (which for CMOS I guess means that there is an amplifier with a high gain value before the readout ADC?). Its noise structure appears to have a very low spatial frequency behaviour since I need a digital image processing filter with a very large kernel to remove it. I don't think this is being caused by typical photon shot noise. Perhaps some form of dark current shot noise or possibly thermal effects?
Any comments, suggestions or links to papers etc would be most welcome. Many thanks!