One of the big advances in image sensor development (both CMOS and CCD) is a reduction in read noise. Read noise is basically the noise added to the pixel when reading out the image sensor. The noise signal is added by the electronic circuits involved in the conversion of charge to the digital signal. With CMOS image sensors this process is all done on chip. With CCD, read noise is added both on-chip and off-chip as analog to digital conversion (ADC) is done off-chip.
Essentially, read noise is the noise floor in the dark part of the image. Therefore, read noise is particularly important in low light applications or light-starved systems but there are other considerations as well for the low light camera to be usable.
There are some applications that require a low noise camera (such as scientific microscope applications) where an extremely low read noise image sensor is implemented, such as EMCCD or Scientific CMOS or even Low-Noise CMOS, but active cooling is also required. The cooling is needed to suppress other noise sources in the image sensor. This is a bulky and costly solution, which is not practical for other applications such as automated microscopy systems or outdoor surveillance.
As mentioned, there are now lower read noise capabilities available in non-scientific CMOS image sensors as well. But, read noise does not provide the full story about the total noise level which is also relevant to know the low light performance. There are a few image sensor companies that pay special attention to suppress other noise sources in order to make CMOS image sensors useful in low light applications without active cooling. This includes reducing the read noise as well as managing the dark current of the image sensor.
Dark Current is the background signal present in the image sensor readout when no light is incident upon the image sensor. Read noise does not tell you about dark current. While dark current is not the same as noise, it is unwanted signal. It does contribute to the total noise level, as there is a resulting shot noise in the dark current.
One of the challenges with dark current is that it increases with temperature. This can mean there is a greater total noise level with increased temperature of the image sensor (which will happen during operation even under indoor environmental conditions). When this is not managed in the image sensor or in the camera (with cooling), a significant increase in noise level occurs even with high performance image sensors. As mentioned, this has been an area of focus in image sensor development to get closer to the ideal image sensor such that all noise levels are below the read noise level of the pixel in order to stay stable over temperature.
Example relationship of total noise (read noise plus shot noise in dark current) with increased image sensor temperature where CMOS sensor B represents the performance of an image sensor with management of dark current.
So when evaluating cameras for low light applications, consider the total noise over temperature and not just the read noise specification provided to get a fuller view of if the camera will provide the necessary performance.