GENERATION, CONTROL, AND MEASUREMENT 



149 



A uniformly heated opaque enclosure or cavity, containing a small 

 opening for sampling the radiant flux, is a nearly perfect complete radi- 

 ator source. The character of the escaping energy is a function only of 

 the temperature of the enclosure and is completely independent of its 

 composition. The flux, radiated by the walls, encounters internal multi- 

 ple reflections, and all the flux is eventually absorbed by the walls except 

 for the small sample that escapes 

 through the opening. The noble 

 metals provide known melting 

 points with which to control the 

 temperature of the cavity. Ex- 

 perimental complete radiators are 

 used principally as radiation stand- 

 ards. Forsythe (1937) describes 

 the construction of several stand- 

 ard complete radiator sources. 



It was shown by Kirchhoff that 

 the capacity of a substance to emit 

 radiant energy is proportional to 

 its ability to absorb that same 

 energy. Thus highly absorbing 

 "black" materials such as carbon 

 are more efficient radiators than 

 bright polished metals such as 

 aluminum and tungsten. If W is 

 the radiant emittance of a body, a 

 the absorptivity for radiant energy 

 of the same spectral distribution, 

 and Wb the radiant emittance of a 

 complete radiator, then W = aWb- 

 For all actual materials a is less 



than 1, and the power radiated at any temperature is always less than 

 that of a complete radiator. 



The relation between the total flux per unit area W and the absolute 

 temperature T for a complete radiator is given by the Stefan-Boltzmann 

 law, 



TT^ = <T^^ (3-14) 



where <r is a constant having the value 5.672 X IQ-^^ w cm-^ deg-^ 

 (DuMond and Cohen, 1948; Illuminating Engineering Society, 1952). 

 Spectral-energy-distribution curves for a complete radiator at various 

 temperatures plotted on a logarithm scale of intensity are given in Fig. 

 3-5. As the temperature increases, the total area under the curve W 

 mcreases rapidly as the fourth power of the temperature, and the wave 



10 



100,000 



100 1000 10.000 



WAVE LENGTH, m^ 



Fig. 3-5. Spectral omission of a black- 

 body radiator at various temperatures 

 plotted on a logarithm scale of wave 

 lengths. As the temperature increases, 

 the wave length of maximum emission 

 Xm along the line A-B shifts to the 

 shorter wave lengths, as predicted by the 

 Wien law. (Froin lES Handbook, 1952.) 



