3. Low light level sensors

For night-time imaging, the very low levels of illumination encountered mean that sensors must be much more sensitive and less noisy than those designed for daytime use, and historically based on photoconduction: photoemission by a photocathode naturally seemed to be the only possible solution. The introduction and ongoing improvement of solid-state circuits now make it possible to exploit the very high quantum efficiency of photodiodes, and pave the way for new low-light devices.

But let's start with vacuum devices featuring a photocathode:

A photocathode is a layer of material which, when placed in a vacuum, emits electrons when it receives light. In the absence of an electric field, these electrons fall back onto the layer. In the presence of an electric field, they can be accelerated, and possibly multiplied, to form a brighter image,...