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A branch of science that deals with the measurement or detection of radiant electromagnetic energy. Radiometry is divided according to regions of the spectrum in which the same experimental techniques can be used. Thus, vacuum ultraviolet radiometry, intermediate-infrared radiometry, far-infrared radiometry, and microwave radiometry are considered separate fields, and all of these are to be distinguished from radiometry in the visible spectral region. Curiously, radiometry in the visible is called radiometry, optical radiation measurement science, or photometry, but it is not called visible radiometry. See Electromagnetic radiation, Infrared radiation, Light, Ultraviolet radiation
Any radiation detector (such as a thermometer) that responds to an increase in temperature caused by the absorption of radiant energy is known as a thermal detector. Similarly, any detector (such as a photochemical reaction) that responds to the excitation of a bound electron is called a photon or quantum detector.
Liquid-in-glass thermometers are sluggish and relatively insensitive. The key to developing thermal detectors with better performance than liquid-in-glass thermometers has been to secure a large and rapid rise in temperature associated with a high sensitivity to temperature changes.
Thermal detectors have been based upon a number of different principles. Radiation thermocouples produce a voltage, bolometers undergo a change in resistance, pyroelectric detectors undergo a change in spontaneous electric polarization, and the gas in pneumatic detectors (Golay cells) and photoacoustic detectors expands in response to incident radiation. The periodic expansion and contraction of the gas in response to high-frequency modulated radiation is detected by a sensitive microphone in the case of the photoacoustic detector. The Golay cell, on the other hand, uses a sensitive photomultiplier and a reference beam of light to detect distortion of a flexible membrane mirror caused by the expansion and contraction of the gas. See Bolometer, Pyroelectricity, Thermocouple
The main problem with thermal detectors is that they respond not only to electromagnetic radiation but to any source of heat. This makes their design, construction, and use rather difficult, because they must be made sensitive to the radiation of interest while remaining insensitive to all other sources of heat, such as conduction, convection, and background radiation, that are of no interest in the particular measurement.
Photon detectors respond only to photons of electromagnetic radiation that have energies greater than some minimum value determined by the quantum-mechanical properties of the detector material. Since heat radiation from the environment at room temperature consists of infrared photons, photon detectors for use in the visible can be built so that they do not respond to any source of heat except the radiation of interest.
Following the introduction of planar silicon technology for microelectronics, the same technology was quickly exploited to make planar photodiodes based on the internal photoelectric effect in silicon. In these devices, the separation of a photogenerated electron-hole pair by the built-in field surrounding the p+n junction induces the flow of one electron in an external short circuit (such as the inputs to an operational amplifier) across the electrodes. The number of electrons flowing in an external short circuit per absorbed photon is called the quantum efficiency. The use of these diodes has grown to the point where they are the most widely used detector for the visible and nearby spectral regions. Their behavior as a radiation detector in the visible is so nearly ideal that they can be used as a standard, their cost is so low that they can be used for the most mundane of applications, and their sensitivity is so high that they can be used to measure all but the weakest radiation (which requires the most sensitive photomultipliers). See Semiconductor diode
Research efforts have been directed at producing photon detectors based on more exotic semiconductors, and more complicated structures to extend the sensitivity, time response, and spectral coverage.