Radiation Pyrometer

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radiation pyrometer

[‚rād·ē′ā·shən pī′räm·əd·ər]
An instrument which measures the temperature of a hot object by focusing the thermal radiation emitted by the object and making some observation on it; examples include the total-radiation, optical, and ratio pyrometers. Also known as noncontact thermometer; radiant-energy thermometer; radiation thermometer.

Radiation Pyrometer


an instrument for measuring the temperature of opaque bodies from the radiation the bodies emit in the optical region of the spectrum. A body whose temperature is to be determined by a radiation pyrometer must be in thermal equilibrium and must have an absorptivity that is close to unity. Common types of radiation pyrometers include brightness, color, and total-radiation pyrometers. Of particular importance is the brightness pyrometer, which provides the highest accuracy of temperature measurements in the range from 103 to 104 °K.

Figure 1. Schematic of a manually operated disappearing filament pyrometer: (1) radiation source, (2) optical system (pyrometer telescope, (3) standard incandescent lamp, (4) narrow-passband filter, (5) objective, (6) rheostat by which the filament current is regulated, (7) measuring instrument (mllllammeter)

A simple example of a brightness pyrometer is the manually operated disappearing filament pyrometer (Figure 1). An objective lens focuses the image of the body being studied in the plane of the filament of a standard incandescent lamp. The filament is viewed against the background of the image through an eyepiece and a red filter, which permits isolation of a narrow region of the spectrum in the vicinity of the wavelength λe = 0.65 micron (μ). By adjusting the current in the filament, the brightness of the filament can be made to match that of the body, whereupon the filament becomes indistinguishable. The scale of the instrument that records the filament current is usually calibrated in degrees Celsius or Kelvin; at the moment of matched brightness, the instrument registers the brightness temperature Tb of the body. The body’s true temperature T is determined on the basis of the thermal radiation laws of Kirchhoff and Planck from the formula

where C2 = 0.014388 m •°K, αλ,T is the body’s absorptivity, and λe is the effective wavelength of the pyrometer.

The accuracy of the result depends first of all on the extent to which the requirements for pyrometric measurement are fulfilled —for example, the requirement that αλ ≈ 1. For this reason, the surface being observed is given the form of a cavity. The principal instrumental error is due to the instability of the temperature lamp. An appreciable error may also result from the individual characteristics of the observer’s eye. This second kind of error is absent in photoelectric pyrometers (Figure 2). The error of model laboratory photoelectric pyrometers does not exceed a few hundredths of a degree at T = 1000°C. The error of commercial mass-produced photoelectric pyrometers is greater by one order of magnitude, and the error of manually operated pyrometers is greater by an additional order of magnitude. Standard brightness pyrometers have been accepted as the basic means for determining temperatures on the International Practical Temperature Scale (IPTS-68) above the freezing point of gold (1064.43°C).

Color pyrometers can be used to measure the temperature of bodies for which α is approximately constant in the optical region of the spectrum. They determine the ratio of brightnesses in, usually, the blue and red bands of the spectrum b11, T)/b22, T)—for example, when λι = 0.48 μ and λ2 = 0.60 μ. The scale of the instrument is calibrated in degrees Celsius and indicates the color temperature Tc. The body’s true temperature T is determined from the formula

Figure 2. Optical system of an automatic photoelectric pyrometer: (1) radiation source; (2) lenses of optical system; (3) modulator, which lets pass radiation alternately from the source and from the standard lamp (4) to the photoelectric cell (7); (5) narrow-passband filter; (6) concave lens. The photoelectric cell is exposed alternately to the source and to the lamp. When the respective brightnesses are unequal, there is produced in the circuit of the photoelectric cell an alternating component of the photocurrent; the amplitude of this component is proportional to the difference of the brightness. When measurements are made, the filament current of the lamp is regulated in such a way that the alternating component of the photo-current becomes equal to zero.

Color pyrometers are less accurate, less sensitive, and more complicated than brightness pyrometers; the two types are used in the same temperature range.

The most sensitive and least accurate radiation pyrometers are total-radiation pyrometers, which record the “total” radiation of the body. Their operation is based on the Stefan-Boltzmann and Kirchhoff radiation laws. The objective of a total-radiation pyrometer focuses the observed radiation on a detector, which is usually a thermopile or bolometer. The detector’s signal is recorded by an instrument that is calibrated with respect to a black body and that indicates the radiation temperature Tr. The true temperature of the body is determined from the formula

where αT is the total absorptivity of the body. Total-radiation pyrometers can be used to measure temperatures beginning at 200°C.

Pyrometers are widely used in industry in temperature monitoring and control systems for a variety of technological processes.


Ribaud, G. Opticheskaia pirometriia. Moscow-Leningrad, 1934. (Translated from French.)
Gordov, A. N. Osnovy pirometrii, 2nd ed. Moscow, 1971.


Radiation Pyrometer


a pyrometer that measures radiation temperatures, that is, a device for determining without contact the temperatures of objects according to their total thermal radiation over the entire range of wavelengths.

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