96 



TABLE 76.— BRIGHTNESS OF BLACKBODY, CROVA WAVELENGTH, 



MECHANICAL EQUIVALENT OF LIGHT, LUMINOUS INTENSITY, 



AND LUMINOUS EFFICIENCY OF BLACKBODY 



The values of the luminous intensity / in candles and the luminous flux F in lumens 

 have been calculated using Planck's equation and the values of the luminosity factors K\ 

 given in Table 58. The basis of these values is the value of the Waidner-Burgess standard 

 of light intensity. 



The following equation is used : 



#o= — fj(\T)Kxd\, 



mirj 



where 5o = 60 candles per cm 2 , T = 2042.16 °K, and m = the minimum mechanical equiva- 

 lent of light expressed in watts per lumen. 



The radiation constants (Table 53) used in these calculations and the value given in the 

 table as the brightness of the blackbody at this temperature (2042.16) give for the reciprocal 

 of the mechanical equivalent of light 680 lumens per watt. This means that 1 watt of 

 radiated energy at about X = 0.555m will give 680 lumens. 



White light has sometimes been defined as that emitted by a blackbody at a temperature 

 of 6000 °K. 



The crova wavelength for a blackbody is that wavelength X,., at which the spectral 

 luminous intensity varies at the same rate as the total luminous intensity varies for a 

 change in the temperature. 



SMITHSONIAN PHYSICAL TABLES 



