Photocathode

A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound. When this is struck by a quantum of light (photon), the absorbed energy causes electron emission due to the photoelectric effect. A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound. When this is struck by a quantum of light (photon), the absorbed energy causes electron emission due to the photoelectric effect. For many years the photocathode was the only practical method for converting light to an electron current. As such it tends to function as a form of 'electric film' and shared many characteristics of photography. It was therefore the key element in opto-electronic devices, such as TV camera tubes like the orthocon and vidicon, and in image tubes such as intensifiers, converters, and dissectors. Simple phototubes were used for motion detectors and counters. Phototubes have been used for years in movie projectors to read the sound tracks on the edge of movie film. The more recent development of solid state optical devices such as photodiodes has reduced the use of photocathodes to cases where they still remain superior to semiconductor devices. Photocathodes operate in a vacuum, so their design parallels vacuum tube technology. Sincemost cathodes are sensitive to air the construction of photocathodes typically occurs after the enclosure has been evacuated. In operation the photocathode requires an electric field with a nearby positive anode to assure electron emission. Photocathodes divide into two broad groups; transmission and reflective. A transmission type is typically a coating upon a glass window in which the light strikes one surface and electrons exit from the opposite surface. A reflective type is typically formed on an opaque metal electrode base, where the light enters and the electrons exit from the same side. A variation is the double reflection type, where the metal base is mirror-like, causing light that passed through the photocathode without causing emission to be bounced back for a second try. This mimics the retina on many mammals.