Photokathode

A photokathode is a cathode that emits electrons when it is illuminated by light. This phenomenon is known as the photoelectric effect. Photokathodes are typically made of materials with a low work function, such as alkali metals or their oxides. The work function is the minimum amount of energy required to remove an electron from the surface of a material. When photons of light strike the photokathode, they transfer their energy to electrons in the material. If the energy of the photon is greater than or equal to the work function of the material, the electron can escape from the surface.

The emitted electrons can then be accelerated and collected by an anode, creating an electric current. This current is proportional to the intensity of the incident light, making photokathodes useful as light detectors. They are a crucial component in many electronic devices, including photomultiplier tubes, phototubes, and image intensifier tubes. Photomultiplier tubes, for instance, use a series of dynodes to amplify the signal from the initial photoemission, allowing them to detect very faint light levels. Phototubes are simpler devices that directly convert light into an electrical signal. Image intensifier tubes use photokathodes to convert incoming photons into electrons, which are then amplified and converted back into photons, producing a brighter image. The spectral response of a photokathode, meaning the range of light wavelengths it is sensitive to, depends on the material used.