Unlike visible light cameras, the pixels used in thermal cameras can vary widely in response to neighboring pixels, and some form of calibration is necessary to normalize the image for best quality. Sometimes it is not possible to completely remove artifacts in a thermal imaging system.
One source of fixed-pattern noise can be the non-uniformity compensation (NUC) terms that are applied to pixels in the array. NUC is a process whereby a series of uniform but noisy frames are averaged for the calculation of an offset correction table. The resulting offsets are applied to each pixel in the array to adjust their response with respect to other pixels. If the number of frames is not enough to average out the noise present in the video, then some residual noise is `burned into` the table. This is why the pattern does not move, and is called fixed-pattern.