Radiometer

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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

radiometer

[‚rād·ē′äm·əd·ər]

(electronics)

A receiver for detecting microwave thermal radiation and similar weak wide-band signals that resemble noise and are obscured by receiver noise; examples include the Dicke radiometer, subtraction-type radiometer, and two-receiver radiometer. Also known as microwave radiometer; radiometer-type receiver.

(engineering)

An instrument for measuring radiant energy; examples include the bolometer, microradiometer, and thermopile.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Radiometer

 

(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.

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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, T_n 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 T_n = 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, T_n ≈ 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 T_n 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.

REFERENCES

Esepkina, 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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.