What's an optical detector? Name the factor which decides how good a detector is.?

Answer 1

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 when illuminated
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)[1]. The next generation of astronomical instruments (see for example Astro-E2) will include Cryogenic detectors. In experimental particle physics

Answer 2

An optical detector is anything that responds to light by changing some physical characteristic that is measurable. The physical change is always caused by the interaction of a photon with an electron, but there are many ways this can occur.

The earliest non-living optical detectors were arguably photographic emulsions, where the photon changes the ionization state of a chemical in the emulsion, forming a latent image. A common chemical used was silver halide, such as silver bromide. Later, reacting the halide in the emulsion with a reducing agent to produce metallic silver developed the latent image.

Living optical detectors, like our eyes, are also chemical in nature but the result is a nerve impulse. The process is reversible and renewable. That is, the optical receptors (rods and cones) are re-sensitized when light is removed from them and de-sensitized if continuously exposed to a constant light stimulus.

Photons contain energy. When absorbed, this energy can be converted to heat. This is the principle behind bolometer optical detectors.

Fast optical detectors, such as are used in television cameras, as photo sensors on assembly lines to detect the presence of product or an operator’s hands, or in laser range finders and target designators used by surveyors and the military all rely on electron photo-emission in a vacuum, or electron-hole pair production in a semiconductor. Both are quantum mechanical effects: a photon is absorbed by and thereby adds energy to a weakly bound electron in an atom. An external electrical field then adds more energy to the newly created photoelectron, or electron-hole pair, and allows the resulting electron current to be measured.

The important characteristics of an optical detector are the wavelengths it responds to, the quantum efficiency in which it converts photons to electrons or electron-hole pair events, the responsivity (output produced for a given optical power input), and the detectivity or D* which is a figure of merit related to the area of the detector and the amount of electrical noise the detector produces.

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Answer 3

optical detector is a device which produces current depending upon the intensity of light falling on that detector