Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:^[1]

• Active pixel sensors (APS) are image sensors. Types include:
  • CMOS APS commonly used in cell phone cameras, web cameras, and some DSLRs.
  • An image sensor produced by a CMOS process is also known as a CMOS sensor, and has emerged as an alternative to Charge-coupled device (CCD) sensors.
• Charge-coupled devices (CCD), which are used to record images in astronomy, digital photography, and digital cinematography. Before the 1990s, photographic plates were most common in astronomy. 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.
• 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.
• Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and infrared photons.^[2]
• LEDs which are reverse-biased to act as photodiodes. See LEDs as Photodiode Light Sensors.
• Optical detectors, which are mostly quantum devices in which an individual photon produces a discrete effect.
• 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.
• Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity. Normally the resistance of Photoresistor ( LDR ) decreases with increasing intensity of light falling on it.^[3]
• 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.
• Quantum dot photoconductors or photodiodes, which can handle wavelengths in the visible and infrared spectral regions.

See also [edit]

• List of sensors
• Optoelectronics

References [edit]

External links [edit]

• Fundamentals of Photonics: Module on Optical Detectors and Human Vision