So in a previous post we talked about effective pixels versus total photosites, i.e. the effective number of pixels in a image (active photosites on a sensor) is usually smaller than the total number of photosites on a sensor. That leaves a small number of photosites that don’t contribute to forming an image. These “extra” photosites sit beyond the camera’s image mask, and so are shielded from receiving light. But they are still useful.

These extra photosites receive a signal that tells the sensor how much dark current (unwanted free electrons generated in the CCD due to thermal energy) has built up during an exposure, essentially establishing a reference dark current level. The camera can then use this information to compensate for how the dark current contributes to the effective (active) photosites by adjusting their values (through subtraction). Light leakage may occur at the edge of this band of “extra” photosites, and these are called “isolation” photosites. The figure below shows the establishment of the dark current level.