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

1. Ag-O-Cs, (silver oxide/caesium, also called S-1). This was the first compound photocathode material, developed in 1929.
2. High temperature bialkali or low noise bialkali (sodium-potassium-antimony, Na-K-Sb). This material is often used in oil well logging since it can withstand temperatures up to 175 °C. At room temperatures, this photocathode operates with very low dark current, making it ideal for use in photon counting applications.
3. GaAs (gallium(II) arsenide). This photocathode material covers a wide spectral response range, from ultraviolet to 930 nm (nm = nanometre, a measure of the wavelength of the light or other electromagnetic radiation).
4. Cs-Te, Cs-I (caesium-telluride, caesium-iodide). These materials are sensitive to vacuum UV and UV rays, but not to visible light and are therefore referred to as solar blind. Cs-Te is insensitive to wavelengths longer than 320 nm, and Cs-I to those longer than 200 nm.

These devices are mostly based on alkali metals.