When hot and warm pixels can be observed in cameras?

The technical term “warm/hot pixel” describes a phenomenon of digital image sensors that can easily be observed when long exposure times are taken (generally > 3s, although this can vary from chip to chip). When images are recorded using long exposure times, single pixels are observed that appear to be much brighter than surrounding pixels. When dark images (no optical input) are recorded using long exposure times, bright pixels are observed at the same locations. These bright pixels are referred to as warm or hot pixels. The term “warm pixels” is generally used to refer to pixels that follow the impinging light, while “hot pixels” refer to pixels don’t follow the impinging light. In either case such hot/warm pixels are typically much different than neighborhood pixels.

Physically, these “warm/hot pixels” are mainly caused by charge leakages within the image sensor chip. Although the warm/hot pixels are randomly distributed within the chip, they are in a fixed position. Under normal conditions (shorter exposure times, normal light), many of them are not visible since their contribution to the general noise level is below this level.

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