Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:[1|http://en.wikipedia.org/wiki/Photodetector#cite_note-0]

• Optical detectors, which are mostly 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, such that the tube conducts a current proportional to the light intensity.
• Phototransistors, which act like amplifying photodiodes.
• 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[2|http://en.wikipedia.org/wiki/Photodetector#cite_note-1].
• Charge-coupled devices (CCD), which are used to record images in astronomy, digital photography, and digital cinematography. Although before the 1990s photographic plates were the most common in astronomy. 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.[3|http://en.wikipedia.org/wiki/Photodetector#cite_note-2] The next generation of astronomical instruments, such as the Astro-E2, include cryogenic detectors. In experimental particle physics, a particle detector is a device used to track and identify elementary particles.
• LEDs reverse-biased to act as photodiodes. See LEDs as Photodiode Light Sensors. http://en.wikipedia.org/wiki/Photodetector