Noise

Noise, in the context of night vision, is an inherent property of any device that processes any type of analog signal, which the image intensifier of an analog night vision device and the optical sensor of digital night vision or thermal device fall under.

As noise reduces the quality of a signal, decades of research have been invested into reducing the amount of noise, which is measured by the signal-to-noise ratio (SNR).

Image Intensifiers

Due to the complexity of image intensifiers they produce several types of noise, mixed together in the output image.

Fixed Pattern Noise (FPN)

Inaccuracies in the manufacturing of the image intensifier result in slight imperfections in the uniformity of the output brightness accross the output image, referred to a fixed pattern noise.

Shot Noise

Shot noise is named after the inventor of the concept, Walter Schottky. In analog night vision, shot noise appears as a kind of transparent TV static overlayed over the image.

To understand how it occurs, it is paramount to understand what happens to the signal depending on the amount of light the image intensifier is exposed to.

When looking at any arbitrary, small point on the input area of the image intensifier, we can count the number of photons entering it in any given small time frame. In bright environments, this number will be extremely large. In very dark environments, the number will become smaller and smaller. Due to chance, however, the number of photons hitting that area in that time frame will differ slightly over time. As the environment becomes darker, this slight difference will become more and more significant, until it becomes visible as that TV like static over the image.

Of course this is a simplification. Image intensifiers will not convert 100% of the photons they receive. As such, lower performing image intensifiers will have more shot noise, while higher performing ones will have less. But there's a theoretical maximum to how high the SNR of the image intensifier can be, as even at a 100% efficiency the effect will still occur in sufficient darkness.

Scintillation

Scintillation is an effect that only occurs in image intensifiers with MCPs. It describes the bright flashes of light constantly occuring at random points over the screen. This is caused by the MCP sometimes "loosing" electrons without any input electron triggering the cascade. As such, this happens even when the input of the image intensifier is completely covered. Scintiallation, like shot noise, will become less pronounces the brighter the environment, as it gets drowned out by the actual signal.