Dark shot noise results from the fact that even if there is no light at all falling on a pixel, the photodiode “faucet” still has a small flow of “leakage” electrons that are generated thermally. When the charge accumulated in a pixel is read out there is no way to tell thermally generated “leakage electrons” apart from photoelectrons. Therefore, this dark current adds uncertainty in the total number of electrons that are measured.

At longer exposures, dark current can fill up more of the full-well-capacity of the “bucket” thereby reducing the available space for photoelectrons. In addition to using up some of the capacity for photoelectrons, there is a temporal variation in the number of dark electrons collected from one exposure to the next. This temporal variation is referred to as dark-shot-noise.

Since it can be modeled as a Poisson distribution, the dark-shot-noise is estimated to be the square root of the number of dark signal electrons that are expected to be captured in each exposure. Since Dark Current is a thermal phenomenon, its effect (Dark Shot Noise) can be mitigated by cooling the image sensor.

$$ \sigma_{\text{dark}} = \sqrt{I_{\text{dark}}} $$

Where:

• $\sigma_{\text{dark}}$: Dark shot noise.
• $I_{\text{dark}}$ [e-]: Mean Dark Signal.