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Either of these artifacts results in a distorted image but can be avoided by using correct lighting; therefore, these artifacts almost never appear in machine vision applications where the lighting is under control.
The uniformity of an image is influenced by the fixed pattern noise present in the image. There are two types of fixed pattern noise: fixed pattern noise in a black image and fixed pattern noise in a white image.
The fixed pattern noise in a black image can further be divided into two types. The first is a difference in the average dark current of the photodiodes. This can differ from pixel to pixel and is visible as a granular effect. This effect does not change from image to image. The second is an excessive difference in dark current of a single photodiode with respect to other photodiodes. This can be seen as white spots and are often referred to as hot pixels.
Both types of fixed pattern noise in a black image depend on gain, integration time, and temperature. For example, the dark current in a pixel doubles with every 6 to 8 degrees Celsius temperature increase.
Fixed pattern noise in white is caused by differences in sensitivities of the pixels and is independent of camera settings. This can observed as dark spots on a white image.