138 BIOLOGICAL EFFECTS OF RADIATION 



multiplied by x. The corresponding values of radiance in a direction 

 normal to the surface for a tungsten filament are shown by dotted lines. ^ 



Table 7 (31) gives the numerical values for black-body radiation 

 from which the curves have been prepared, together with those for a 

 tungsten filament. The values of emissivity upon which the data for 

 tungsten depend are incomplete and approximate, those for 2800°K.2 

 being based entirely upon extrapolation. A more adequate basis for 

 these values is urgently needed. 



It may be shown that Stefan's law, mentioned above, follows as a 

 consequence of Planck's equation (by adding up the effects for all wave- 

 lengths). For the condition that /x be maximum, we obtain the simple 

 relation 



\mT = const. (0.2884 cm.-deg.) 



Thus, the higher the temperature of the body, the shorter the wave- 

 length of maximum radiation. For 6000°K., the approximate tempera- 

 ture of the sun, the maximum intensity occurs in the visible at about 

 0.48m; for 3000°K., a common temperature for a tungsten filament 

 lamp, it occurs in the near infra-red, at 0.96ai; for 1000°K., at 2.88 m; 

 for 300'^K., approximate room temperature, at 9.6/i. 



For convenience in visualizing the shift of maximum and the relative 

 distribution, one may refer to the center section of Fig. 2. 



Thus, since the radiation from a black body is predictable from its 

 temperature, such a black body may be made the standard of radiation 

 for all wave-lengths. To determine the intensity of any other source of 

 radiation, it is only necessary to make a comparison of the radiation for 

 that wave-length with that of a black body. If the source of radiation 

 is compared, wave-length by wave-length, with the black body, one need 

 not use a black-body detector. The difficulty which this method has 

 presented arose from the fact that black bodies were not available of 

 such a high temperature as to radiate with sufficient intensity in the 

 short-wave-length ranges. 



A black-body source consisting of a hollow enclosure immersed in 

 freezing platinum has been developed for use as an absolute standard of 

 candle power by the National Bureau of Standards (66). This provides 

 a black-body source at 2046°K., the fixed point at which pure platinum 

 freezes. By using freezing iridium (38, 67) instead of platinum, a source 



^ Note: The conventional use of the centimeter for wave-length in J\ should be 

 particularly noted. When the smaller customary units are used, values indicated 

 must be multiplied by the corresponding factor: for Angstroms by 10-«; for milli- 

 microns by 10"^; for ^ by 10~*. 



2 K refers to absolute temperature. When not otherwise specified, temperature 

 refers to centigrade scale, C 



