[1] One is the dark signal non-uniformity (DSNU), which is the offset from the average across the imaging array at a particular setting (temperature, integration time) but no external illumination and the photo response non-uniformity (PRNU), which describes the gain or ratio between optical power on a pixel versus the electrical signal output.
In practice, a long exposure (integration time) emphasizes the inherent differences in pixel response so they may become a visible defect, degrading the image.
It is not expressed in a random (uncorrelated or changing) spatial distribution, occurring only at certain, fixed pixel locations.
FPN is commonly suppressed by flat-field correction (FFC) that uses DSNU and PRNU to linearly interpolate and reduce the local photo response (non-uniform PRNL) to the array average.
The main challenge is to generate a flat-field illumination for short time exposures and wavelengths, to avoid speckle (in monochromatic light conditions) and statistical fluctuations of the light stream that become most obvious at short integration times.