Table 159 (continued) 



449 



RELATIVE SPECTRAL LUMINOSITY FOR THE HUMAN EYE AT VARIOUS 



LUMINANCE LEVELS 



In many cases different observers do not yield identically the same values of y\ even 

 if their color vision is normal.* However, the mean value of y\ yielded by a large number 

 of observers having normal color vision provides a reference datum of great utility. 



In conjunction with y\ it is necessary to consider some other related quantities, the 

 first of which is K mai = maximum attainable luminous efficiency, a function of luminance, 

 B, to which the observer's eyes are adapted, and defined as luminous flux (in lumens) 

 emitted per unit radiant flux (in watts) at wave length \ m , at the given adaptation level 

 of luminance. Values of K ma „ for various values of B are given in Table 159 B, based 

 on Weaver's data. 1 In adopting this table we take as the best value of K ma » for high 

 adaptation levels of luminance (0.55 footlambert or more; photopic vision) the quantity 

 680 (new) lumens/watt. 7 This datum is based on the following values of constants 

 for Planck's radiation formula, viz, Ci = 3.740 X 10~ B erg cm. 2 per second, and d = 1.438 

 cm. deg., where the luminance of a blackbody radiator at the temperature of freezing 

 platinum (2042° K.) is taken to be 60 candela (new candles) per square centimeter. 



The following relationships and terminology obtain, where the indicated units are in 

 the m.k.s. system: 



CO 



(1) N = \ N\d\, radiance (watts per steradian per square meter) ; 



J o 



(2) 



(3) 

 (4) 



(5) 



3>x N\ d\ 



y = - 



, relative luminous efficiency or relative luminosity ; 



Nxd\ 

 o 



K\ = Kmax y\, absolute spectral luminosity (lumens/watt) ; 



(oo .oo 



y\ N\d\= K\ N\d\ = K N, luminance (lumens per steradian per 

 o Jo 



square meter) ; 

 K max y\ N\ d\ 



K = 



B 



= — , absolute luminous efficiency or absolute luminosity 



N x d\ 

 



(lumens/watt). 



It should be noted that y\ is a function of X and the luminance B to which the ob- 

 server's eyes are adapted, while K max is a function of B alone. 



If h and n are the spectral transmittance and spectral reflectance, respectively, and 

 H\ is the spectral irradiance of surface (watts/m. a per millimicron), for specified angular 

 (spatial) conditions of irradiation, then the luminous transmittance, T, and luminous 

 reflectance, R, of non-fluorescent materials under these conditions are given by the 

 equations 



(6) 



(7) 



R 



luminous transmittance, 



luminous reflectance. 



T Sears, F. W., Principles of Physics, III, Optics 1949. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



