Radiance thermometers

Principles of Radiation Thermometers. A detailed discussion on opto-electronic temperature measuring systems for radiance thermometers can be found in Ref. 48. In the USA, ITS-90 above the gold point is maintained by NIST [49]. A classical radiation thermometer is shown in Fig. 16.24. Radiation from the object whose temperature is to be measured is... 

Temperature you will probably find several thermometers in the home. High temperatures can be estimated from the colour of the radiance as seen in a dark room. 

Reliability of Radiation Thermometers. Calibration of radiation thermometers at NIST is accomplished by focusing the radiance sensor at a blackbody furnace with known temperature. This blackbody furnace is previously calibrated by comparison calibration against a standard lamp, which, in turn, is calibrated at the gold point [52], With calibration performed at NIST, the accuracy of a radiation thermometer is within 0.4°C at the gold point, within about 2°C at 2200°C and about 10°C at 4000°C. 

The major problem in using a single wavelength radiation thermometer to measure the surface temperature is the unknown emissivity of the measured surface. The emissivity is the major parameter in the spectral radiance temperature equation (Eq. 16.28) for the temperature evaluation. Objects encountered for temperature measurements are often oxidized metal surfaces, molten metal, or even semitransparent materials. On these surfaces, the emissivity is usually affected by the surface temperature and the manufacturing process for these materials. 

To overcome the problems faced by the single-wavelength radiation thermometer and the ratio pyrometer, a double-wavelength radiation thermometer (DWRT) measures the spectral radiance itself at two distinct wavelengths for surface temperature evaluation. For this method to be used, the emissivity compensation function e i = fl v) must be defined. A detailed description of the principle for DWRT can be found in Ref. 53. When the emissivity relation x.i = Ae ) at two distinct wavelengths e i = fl v) is established, the true temperature on the measured surface can be determined from the inferred temperature, which is defined as... 

Table VIII furnishes, not emissivity values but the differences between real temperatures measured with a standard thermometer at the very instant when the radiance temperature was recorded by a radiometer operating in the 10.5-12.5 m band (Pouquet, 1972, Rev. Geom. Dyn.).

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