a device whose change in state or response under the action of a flux of optical radiation is used to detect the radiation, to measure the radiation, or to record and analyze images of the radiating object. Optical detectors constitute the broadest class of radiation detectors. In an optical detector radiant energy in the optical range is converted into other forms of energy.

The four important parameters characterizing the properties and capabilities of various types of optical detectors follow.

(1) The threshold sensitivity is the minimum radiation flux that can be detected against the background of the inherent noise of the detector.

(2) The conversion factor, or relative sensitivity, relates the radiation flux incident on the detector to the magnitude of the signal at the detector output.

(3) The time constant is the time required for a signal at the output of the detector to rise to a specified level. This parameter is used as a measure of the capacity of the detector to register optical signals of short duration.

(4) The spectral response expresses the dependence of the detector’s sensitivity on the radiation wavelength.

Detectors whose sensitivity depends weakly on wavelength over a range of wavelengths are called nonselective, in contrast to selective detectors, which have clearly defined maxima and (or) minima in their spectral response.

Optical detectors are classified as thermal, photoelectric, mechanical, and chemical. Thermal detectors include thermocouples, metal and semiconductor bolometers, molecular radiometers, and optic-acoustic detectors. Thermocouples and vacuum bolometers are the most widespread types of thermal detectors. A change in the temperature of its light-absorbing surface produces a thermoelectromotive force in the thermocouple. Thermopiles, which are series connections of several thermocouples, have a greater sensitivity. Optic-acoustic (pneumatic) detectors register the increase in volume of a gas heated by absorbed radiation. Thermal detectors used in the infrared (IR) range include liquid crystals, which change color when they are heated by radiation. Thermal detectors are usually nonselective and are suitable for measurements of radiant energy over a wide spectral range: from 200 nanometers (nm) to 20 microns (μ), sometimes up to 1,000 μ. The threshold sensitivity of the best thermal detectors is between ~ 10^–10 and 10^–11 watt (W). The time constant in most cases is between 10^–1 and 10^–3 sec.

Photoelectric detectors are divided into detectors based on the photoconductive effect and detectors based on the photoemissive effect. Photoelectric detectors include photocells, photomulti-plier tubes, photoresistors, photodiodes, image converters, detectors based on the photoelectromagnetic effect, and optical-region quantum amplifiers. These detectors are selective, and their response depends on the magnitudes of the energies of the individual absorbed photons. The spectral sensitivity of photoemissive detectors has a characteristic longwave (“red”) limit in the 0.6 to 1.2 μ region; the limit is determined by the nature of the detector material. The threshold sensitivity of photoemissive detectors can be reduced to 10^–12 to 10^–15 W with a time constant of less than 10^–9 sec, and for image converters the time constant can reach 10^–12 sec. The sensitivity of photon counters is still higher—up to 10^–17 W/sec. The advantage of photoresistors, photodiodes, and detectors based on the photoelectromagnetic effect over photoemissive detectors lies in their ability to operate in the far IR region of the spectrum (10 to 30 μ). The limiting sensitivity of photoresistors in a frequency band 1 Hz wide is 10^–10 to 10^–12 W with a time constant of 10^–5 to 10^–7 sec. A new form of detector has been developed in the USSR for recording ultrashort pulses of laser radiation in the IR region; it makes use of the effect of entrainment of free electrons in semiconductors by photons. When light is absorbed by electrons, the momentum of the incident light wave is absorbed along with its energy. The redistribution of the momentum between the crystal lattice of the semiconductor and the free electrons causes an ordered motion (entrainment) of the electrons relative to the lattice and is recorded in the form of a current or electromotive force. Detectors of this type have a high time resolution with a time constant of ~10^{- 11} to 10^–10 sec. They do not require forced cooling or the use of power supplies. Mechanical (ponderomotive) detectors are usually made in the form of torsion balances and respond to the pressure of light. They are used comparatively infrequently, since they are very sensitive to vibrations and to various thermal processes.

Photochemical detectors include all the types of photosensitive films used in modern photography. Unlike thermal and photoelectric detectors, a photosensitive film adds up the photochemical action of the radiation. Moreover, the radiation energy is measured directly from the optical density of the blackening of the layer. The eyes of living creatures can also be regarded as optical detectors. The spectral region in which the human eye is sensitive lies between 0.4 and 0.8 μ and is called the visible region. The human eye is a selective light detector with maximum sensitivity at about 555 nm. The dark-adapted human eye has a threshold sensitivity of 10^–17 W/sec, which corresponds to several tens of photons per second. The eyes of other living creatures—such as mammals, birds, fish and insects—exhibit a great variety of properties. For example, the eyes of certain insects respond to the polarization of light.