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Fixed pattern noise (FPN)

If the output of an image sensor under no illumination is viewed at high gain a distinct non-uniform pattern, or fixed pattern noise, can be seen. This fixed pattern can be removed from the video by subtracting the dark value of each pixel from the pixel values read out in all subsequent frames. Dark fixed pattern noise is usually caused by variations in dark current across an imager, but can also be caused by input clocking signals abruptly starting or stopping or if the CCD clocks do not closely match one another.

Mismatched CCD clocks can result in high instantaneous substrate currents, which, when combined with the fact that the silicon substrate has some non-zero resistance, can cause in the substrate potential bouncing.
The pattern noise can also be seen when the imager is under uniform illumination. An imager which exhibits a fixed pattern noise under uniform illumination and shows no pattern in the dark is said to have light pattern noise or photosensitivity pattern noise. In addition to the reasons mentioned above, light pattern noise can be caused by the imager becoming saturated, the non-uniform clipping effect of the anti-blooming circuit, and by non-uniform, photosensitive pixel areas often caused by debris covering portions of some pixels. 

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