A detector is a device that detects or measures some phenomenon or stimulus, and produces some signal in response. More generally, the term may be applied to biological organs that perform a similar function. The eye for example converts the energy of those photons reaching the retina into electrical signals which are further processed by the brain.

Most optical detectors are quantum devices in which an individual photon produces a discrete effect. See also: List of optical topics Optics (appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... In physics, the photon (from Greek φως phos, meaning light) is the quantum of the electromagnetic field, for instance light. ...

• In a photographic plate, 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.
• In electronic detectors, the photon excites an electron:
  • into the conduction band (in semiconductor detectors)
  • into vacuum (in photomultiplier tubes)
  • in a superconductor, generating a transient heat pulse that breaks a large number of Cooper pairs (see Josephson effect).

There are 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). Photographic plates were one of the earliest forms of photographic film, in which a light-sensitive emulsion of silver salts was applied to a glass plate. ... A silver halide is one of the compounds formed between silver and one of the halogens, usually silver bromide (AgBr), silver chloride (AgCl) and silver iodide (AgI). ... In film developing, photographic developer (or just developer) is a chemical that makes the image on the film or print visible. ... A semiconductor particle detector is a device that uses a semiconductor (usually silicon) to detect the passage of charged particles. ... Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... Superconductivity is a phenomenon occurring in certain materials at low temperatures, characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect. ... BCS theory successfully explains conventional superconductivity, the ability of certain metals at low temperatures to conduct electricity without resistance. ... The Josephson effect is named after British physicist Brian David Josephson who predicted its existence in 1962. ... Pyroelectricity is the ability of certain materials to generate an electrical potential when they are heated or cooled. ... In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal energy into electrical energy. ... A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ... Introduction Cryogenic detectors are detectors that operate usualy at (very) low temperatures, well below 77 degrees Kelvin the liquefaction temperature of nitrogen. ...

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)[1]. 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. Lunar astronomy: the large crater is Daedalus, photographed by the crew of Apollo 11 as they circled the Moon in 1969. ... A specially developed CCD used for ultraviolet imaging in a wire bonded package. ... A semiconductor particle detector is a device that uses a semiconductor (usually silicon) to detect the passage of charged particles. ... Photographic plates were one of the earliest forms of photographic film, in which a light-sensitive emulsion of silver salts was applied to a glass plate. ... Eastman Kodak Company NYSE: EK is a large multinational public company producing photographic materials and equipment. ... The Astro-E2 satellite was a joint project between scientists from NASA and Japan Aerospace Exploration Agency (JAXA). ... Introduction Cryogenic detectors are detectors that operate usualy at (very) low temperatures, well below 77 degrees Kelvin the liquefaction temperature of nitrogen. ... Particles erupt from the collision point of two relativistic (100GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... In experimental particle physics, a particle detector is a device used to track and identify high-energy particles, such as produced by nuclear decay, cosmic radiation, or reactions in a particle accelerator. ...

In an AM radio receiver, a "detector" is a circuit or device that recovers the signal of interest from the modulated wave. This device is usually based on a diode, which converts the (usually pre-amplified) incoming high-frequency (many kHz) AC signal into a "DC" (low frequency AC) signal. The latter has noise remaining, but its envelope is the required audio frequency output. In a FM radio, a circuit called a discriminator is used to convert frequency variations to amplitude variations. Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. ... An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ... In information theory, a signal is a flow of information. ... ... NOiSE is a one volume manga created by Tsutomu Nihei as a prequel to his acclaimed ten-volume work, Blame!. It offers some rather sketchy information concerning the Megastructures origins and initial size, as well as the origins of Silicon life. ... An envelope detector is a device which is used to demodulate AM signals. ... Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

References

• Dalven, Richard (1980). Introduction to Applied Solid State Physics, Plenum. ISBN 0-306-40385-4.
• Enss, Christian (Editor) (2005). Cryogenic Particle Detection, Springer, Topics in applied physics 99. ISBN 3-540-20113-0.
• Leo, William R. (1987). Techniques for Nuclear and Particle Physics Experiments : A How-to Approach, Springer-Verlag. ISBN 0387173862.
• http://www.ceia.net