TABLE 77.— OPTICAL PYROMETER 97 



An optical pyrometer is a device for measuring the temperature of a high-temperature 

 radiating body by comparing its brightness for a selected wavelength interval (within 

 the visible spectrum to be sure) with that of some standard selected source. The wave- 

 length, or wavelength interval, is generally selected by the use of a red glass in the eye- 

 piece. This gives rise to the term effective wavelength. (See Table 562.) The effective 

 wavelength of a monochromatic screen for a definite temperature interval has been defined 

 as the wavelength for which the relative brightness, as calculated from Wien's equation 

 for this temperature interval, is the same as the ratio of the integral luminosities for these 

 two temperatures, as measured through the red screen. 



Various devices are used to make these comparisons, and different devices have been 

 used as the comparison source. It seems that most users of the optical pyrometer today 

 prefer to use the disappearing-filament type, which has a small filament as the comparison 

 source. 



The optical pyrometer as generally calibrated gives the true temperature of blackbodies 

 but not of other radiators. If one radiating characteristic of any other radiator — e.g., its 

 emissivity — is known, true temperatures can be determined of such radiators, e.g., an 

 incandescent tungsten filament, by the use of the optical pyrometer. The emissivities of a 

 number of sources are given in Table 78. 



The true temperature T of a non-blackbody may be determined from its brightness 

 temperature, S» (the apparent temperature), and its emissivity c\ from the following 

 relation : 



J 1__ X log c\ 



T S\ c 2 log e 



For some calculated values see Table 79. 



This entire subject is extensively treated in "Temperature, Its Measurement and Con- 

 trol," a report of a symposium on this subject published by the Reinhold Publishing Co. 

 in 1941. 



SMITHSONIAN PHYSICAL TABLES 



