TABLES 303-305. 



2OI 



BRIGHTNESS OF BLACK BODY. CROVA WAVE-LENGTH. MECHANICAL EQUIVALENT 

 OF LIGHT. LUMINOUS INTENSITY AND EFFICIENCY OF BLACK BODY- 



The values of L, the luminous intensity, are given in light watts/steroradian/cm 2 of radiating surface 

 = (I/TT) J^ V^E^dX, where V^ is the visibility of radiation function. 



Mechanical equivalent. The unit of power is the watt; of lumininous flux, the lumen. The ratio of these two quan- 

 tities for light of maximum visibility, X = 0.556 ,K the stimulus coefficient Vm; its reciprocal is the (least) mechanical 

 equivalent of light, i.e., least since applicable to radiation of maximum visibility. A better term is umi 

 lent of radiation o maximum visibility." One lumen =0.001496 watts (Hyde, Forsythe, Cady); or i w 

 tion of maximum visibility (X = 0.556 n) = 668 lumens. 



White light has sometimes bee i denned as that emitted by a black body at 6000 K. 



The Crova wave-length for a black body is that wave-length, X, at which the luminous intensity varies bv the 

 same fractional part that the total luminous intensity varies for the same change in temperature 



TABLE 303. Brightness, Crova Wave- 

 length of Black Body, Mechanical 

 Equivalent of Light.* 



TABLE 304. Luminous, Total Intensity and 

 Radiant Luminous Efficiency of Black Body.* 



1919. 



Hyde, Forsythe, Cady, Phys. Rev. 13, p. 45, 



* Coblentz, Emerson, Bui. Bureau of Standards, 14, p. 255, 

 1917- 



NOTE. Minimum energy necessary to produce the sensation of light: Ives, 38 X iQ' 

 Reeves, 19.5 X lo" 10 ; Buisson, 12.6 X io~ 10 erg. sec. (Buisson, J. de Phys. 7, 68, 1917.) 



Russell, 7.7 X 



TABLE 305. Color of Light Emitted by Various Sources.* 



* Jones, L. A., Trans. HI. Eng. Soc., Vol. 9 (1914). 



SMITHSONIAN TABLES. 



