132 



BIOLOGICAL EFFECTS OF RADIATION 



unless both source and observer are specified. A watt of radiant power 

 of a wave-length giving rise to a blue sensation when observed will not 

 appear so bright to the eye as a watt expended at a wave-length which 

 appears yellow. Thus, equal luminous flux does not imply equal radiant 

 power for different wave-lengths. Since different sources of light emit 

 different amounts of power in different wave-lengths, the luminous 

 equivalents will depend upon the wave-length distribution of power. 

 As a first step toward relating these two systems of units, the relative 

 luminous efficiency of different spectral regions has been determined 

 for a large number of observers. A standard relative-visibility curve 

 has been defined which closely approximates the observed average. 

 Table 4 gives the standard relative visibility agreed upon at the last 

 International Commission on Illumination. 



Note: The original article contains interpolations to 1 m/i. 



In specifying relative visibility, the maximum is taken as unity. This 

 maximum occurs at 555 millimicrons or 0.555 n, as will be seen from the 

 curve Fx in Fig. 33. In order to obtain the absolute visibility, or the 

 ratio of luminous flux to radiant power or flux, Fx = L-^/P-k, it is only 



