Photocathode

A photocathode is a negatively charged electrode in a light detection device. They are the main type of photomultiplier. That means they take a little light, and make more of it.
There are instruments which need to magnify the amount of light coming in. Examples are astronomical telescopes and military night-sight equipment: binoculars and telescopes on helmets and rifles etc.
The lens of the telescope or binocular passes the light onto a layer of glass coated with a special light-sensitive metal. When this is struck by light, the absorbed energy causes electrons to jump off. This is called the 'photoelectric effect'. The freed electrons are then collected to produce the final image.
Some photocathode materials.
These devices are mostly based on alkali metals.