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Image sensors: Readout noise

The readout noise is the basic noise that you get with each shot, independent of brightness or exposure.

As explained in the >theory, an exposure series can be used to isolate the noise readout in each image and splitting it into conversion gain and readout noise. It is most important to make sure you are still operating in the linear range of the sensor.

This is the noise of a MT9M001 sensor with its 10 bits encoded in the MSBs of 16 bits.

The diagram shows the squared noise is indeed linear to the brightness. At zero brightness, the squared noise is 1213 DN, so the readout noise is 34.8 DN. The slope is 0.49 e- per DN, so we have around 17 e- noise.

The noise in DN tells very obvious what the impact on a specific image is, but DN depend on the sensor gain settings and pixel encoding. The noise in e- is independent of that, but rather hard to imagine.

It is always a good idea to run a 2D FFT of a noise image to see the noise structure. The image below is typical for many CMOS sensors: Much column noise (horizontal bar) and some row noise (thin vertical bar). It would be bad if they would dominate the overall noise (background). In case there is shading towards the edges, it is likely that a high frequency filter is active and you are not looking at the raw pixel values. A bright ball in the middle would likely indicate a gradient or other low frequency error in the setup.