Thermal Noise
An additional, stochastic source of electrons in a CCD well is thermal energy.
Electrons can be freed from the CCD material itself through thermal vibration
and then, trapped in the CCD well, be indistinguishable from "true"
photoelectrons. By cooling the CCD chip it is possible to reduce significantly
the number of "thermal electrons" that give rise to thermal noise or
dark current. As the integration time T increases, the
number of thermal electrons increases. The probability distribution of thermal
electrons is also a Poisson process where the rate parameter is an increasing
function of temperature. There are alternative techniques (to cooling) for
suppressing dark current and these usually involve estimating the
average dark current for the given integration time and then subtracting
this value from the CCD pixel values before the A/D converter. While this does
reduce the dark current average, it does not reduce the dark current
standard deviation and it also reduces the possible dynamic range
of the signal.