All night vision devices with an image intensifier tube have an optic at the front that collects the light and sends it to an image converter. Here there is a photocathode, which collects the incoming photons (these are the light particles) and generates energy from them in the form of electrons.
These electrons are accelerated massively, and shot onto a phosphorus screen. This releases energy in the form of light, i.e. photons, which is heavily amplified compared to before. Under no circumstances should night vision devices be used in daylight, otherwise the devices can be seriously damaged.