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radiometer(rā'dēŏm`ətər), instrument for detection or measurement of electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
..... Click the link for more information. ; the term is applied in particular to devices used to measure infrared radiationinfrared radiation,
electromagnetic radiation having a wavelength in the range from c.75 × 10−6 cm to c.100,000 × 10−6 cm (0.000075–0.1 cm).
..... Click the link for more information. . One of the earliest experiments in radiometry was performed c.1800 by W. Herschel, who observed the heating of a mercury thermometer by sunlight; he was also able to detect heat radiated from hot but not incandescent bodies. E. Becquerel was able (c.1843) to detect near-infrared radiation by photographic means. Radiometers that function by an increase in the temperature of the device, such as Herschel's thermometer, are called thermal detectors. Commonly used thermal detectors include the thermocouple, which produces a voltage when heated, and the bolometerbolometer
, instrument for detecting and measuring radiation, e.g., visible light, infrared radiation, and ultraviolet radiation, in amounts as small as one millionth of an erg. The bolometer was invented in 1880 by Samuel P. Langley.
..... Click the link for more information. , which changes in electrical resistance when heated. Devices that can, in principle, detect a single quantum of radiant energy, such as Becquerel's photographic plate, are called quantum detectors. Many current quantum detectors are based on the photoelectric cellphotoelectric cell
device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material.
..... Click the link for more information. . The term radiometer is often used to refer specifically to a type of thermal detector invented by Sir William Crookes (c.1874). Because his device was somewhat insensitive and not readily calibrated, it is rarely used today as a scientific instrument. A Crookes radiometer consists essentially of two parts. The first part is a glass bulb from which most of the air has been removed, creating a partial vacuum. The second part is a rotor that is mounted on a vertical support inside the bulb. The rotor consists of four light, horizontal arms mounted at right angles to one another on a central pivot; the rotor can turn freely in the horizontal plane. At the outer end of each arm is mounted a metal vane, placed vertically. Each vane has one side polished and the other blackened; the vanes are arranged so that the polished side of one faces the blackened side of the next. When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surfaces, most of it is absorbed, raising the temperature of the surfaces. The air near a blackened surface thus becomes hotter, exerts a greater pressure on the blackened surface, and causes the rotor to turn. The rate of rotation provides an indication of the intensity of the radiation.
radiometer(ray-dee-om -ĕ-ter) A device that measures the total energy or power received from a body in the form of radiation, especially infrared radiation. In radio astronomy, a radiometer measures the total received radio noise power.
(1) An instrument for measuring the energy of electromagnetic radiation on the basis of the thermal action of the radiation. The uses of such instruments include the study of infrared radiation and solar radiation. Examples are the acti-nometer and pyrheliometer.
(2) The receiving apparatus of a radio telescope.
(3) An instrument for measuring the pressure of acoustic radiation. Such a device is called an acoustic radiometer.
in radio astronomy, a device for measuring the intensity of weak radiation at radio wavelengths (0.1 mm to 1,000 m). In radio telescopes, it functions as a receiver, and in IR radar, it is used in compiling temperature maps of the earth’s surface.
The intensity of the radiation falling on the input antenna of a radiometer is assumed to give an equivalent temperature T of the radiation, which is defined by the Rayleigh-Jeans law: p = kTΔf(k = 1.38 × 10–23 watt/hertz-degree [the Boltzmann constant], and Δf is the bandwidth of the received frequencies). In this case, the sensitivity of the radiometer, that is, the minimum input temperature differential ΔT that the instrument is capable of indicating is defined by the expression
where τ is the signal’s storage time, Tn is the equivalent input-noise temperature, which characterizes the radiometer’s inherent noise level, and α is a coefficient of the order of unity, dependent on the radiometer design. The parameter is often called the radiometric gain; the radiometer is capable of detecting signals that are q times smaller than its inherent noise.
The modulation type of radiometer is the most widespread. In this design, the receiver uses a modulator to switch periodically between the antenna and a dummy, for example, a small antenna pointed at a “cold” region of the sky. In this way, the constant noise component is excluded and the useful signal is discriminated; after amplification, detection, and analog-digital inversion, the useful signal is fed into a computer. The radiometer design is usually based on a superheterodyne or straight receiver. In order to lower input noise, modern radiometers use low-noise parametric amplifiers or masers. Typical radiometer parameters are Tn = 100°K, Δf = 108 hertz, τ = 1 sec, and α = In this case, the sensitivity ΔT is 1.4 × 10–2 °K. By cooling the input amplifiers to the temperature of liquid helium, Tn ≈ 20° K may be attained, and at Δf = 109 hertz, ΔT ≈ 10–3°K.
For a radio telescope-radiometer system located on the earth’s surface, further reduction in Tn and, correspondingly, in ΔT is limited by the noise of emission from the sky (of atmospheric and cosmic origin), which reaches a minimum of 10° K at centimeter wavelengths.
REFERENCESEsepkina, N. A., D. V. Korol’kov, and Iu. N. Pariiskii. Radioteleskopy i radiometry. Moscow, 1973.
Nikolaev, A. G., and S. V. Pertsov. Radioteplolokatsiia. Moscow, 1964.
D. V. KOROL’KOV