CCDs used for low light level applications require a long integration time and/or cooling to have acceptable SNR. Standard commercially available CCD cameras (e.g., for TV applications) are used with a relatively short integration time (≈1/60 s per field) and are operated at room temperature. Consequently, these cameras will have poor SNR if used for low light level applications. The addition of successive frames from a standard CCD camera for the improvement of the SNR is considered here. We show, using a simple model, that a range of signal levels (as low as a few millilux) exists for which frame addition is a viable alternative to long integration times and cooling.
Mark J. O'Malley,
"Charge-coupled devices: frame adding as an alternative to long integration times and cooling," Optical Engineering 31(3), (1 March 1992). http://dx.doi.org/10.1117/12.56091