Dark current is a characteristic value of image sensors. As can be seen from the name, the dark current is the current that flows in each pixel
in the unexposed state. The dark current is strongly temperature-dependent, it is caused by the oscillation energy of the silicon structure and is expressed
. A temperature increase of 8 °C results in approximately a doubling of the dark current. The electron charge caused by the dark current is collected in the potential wells of CCD sensors. This means that the charge of the potential wells increases continuously even in the unexposed state, theoretically until the potential wells cannot accept any more electrons. To prevent this continuous charging of the potential wells, the temperature-induced dark current is reduced by cooling the temperature of the CCD sensor until the photodiodes no longer emit any free electrons.