Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:
Most optical detectors are quantum devices in which an individual photon produces a discrete effect.
* Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
* Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity
* Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated
* Photodiodes which can operate in photovoltaic mode or photoconductive mode
* Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
* Phototubes containing a photocathode which emits electrons when illuminated and in general behaves as a photoresistor.
* Phototransistors incorporating one of the above sensing methods
* Optical detectors that are effectively thermometers, responding purely to the heating effect of the incoming radiation, such as pyroelectric detectors, Golay cells, thermocouples and thermistors, but the latter two are much less sensitive.
* Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and near infra-red photons (Enss 2005).
In astronomy, the detecting devices generally used to record images are charge-coupled devices (CCD, a special semiconductor detector), although before the 1990s photographic plates were the most common. Glass-backed plates were used rather than film, because they do not shrink or deform in going between wet and dry condition, or under other disturbances. Unfortunately, Kodak discontinued producing several kinds of plates between 1980 and 2000, terminating the production of important sky surveys. See, for example, T. M. Girard et al, Astronomical Journal, 127, 3060 (May, 2004). The next generation of astronomical instruments (see for example Astro-E2) will include Cryogenic detectors. In experimental particle physics, a particle detector is a device used to track and identify elementary particles.