All electronic circuitry generates undesirable noise. The effect of this noise on performance is described by the signal-to-noise ratio (SNR). Photon noise, preamplifier noise and dark current noise are the three primary sources of noise in a CCD camera.
Photon noise, also known as photonic or photon shot noise, is a fundamental property of the quantum nature of light. The total number of photons emitted by a steady source over any time interval varies according to a Poisson distribution. The charge collected by a CCD exhibits the same Poisson distribution, so that the noise is equal to the square root of the signal. Photon noise is unavoidable and is always present in imaging systems; it is simply the uncertainty in the data.
Preamplifier noise, also called read noise, is generated by the on-chip output amplifier. This noise can be reduced to a few electrons with the careful choice of operating conditions.
Dark current, or thermally generated charge, can be measured and subtracted from data, but its noise component cannot be isolated. Dark current noise is of particular concern in low light applications.