448 Table 159 



RELATIVE SPECTRAL LUMINOSITY FOR THE HUMAN EYE AT VARIOUS 



LUMINANCE LEVELS* 



Weaver 1 has published provisional data on relative spectral luminosity (yx) for 



observer adaptation to various values of luminance 2 covering a wide range, and his 

 tabulation forms the basis for Table 159-A. 



Relative spectral luminosity is inversely proportional to the spectral radiance re- 

 quired to produce for a normal observer a constant brightness sensation throughout 

 the spectrum. The values are functions of wave length (X) of the radiation and of 

 the luminance (B) to which the observer's eyes are adapted. By definition, the maxi- 

 mum value of the relative spectral luminosity is unity. 



Definitions: 8 *' 5 



N = radiance of a source 



= radiant flux radiated by unit projected area of the source into unit solid angle 

 (m.k.s. unit: watts per steradian per square meter). (Note: N refers to 

 radiant flux over the entire range of wave lengths.) 

 N\ = spectral radiance of a source 



= radiant flux radiated at wave length X by unit projected area of the source 

 into unit solid angle per unit length used in wave length specification 

 (m.k.s. unit: watts per steradian per square meter per millimicron of wave 

 length). 

 B = luminance of a source (formerly called "brightness") 



= luminous flux (lumens) emitted by unit projected area of the source into 

 unit solid angle (m.k.s. unit: lumens per steradian per square meter; or 

 candle per square meter). The c.g.s. unit is the candle per square centimeter. 

 Other relationships : 1 candle/m. 2 = 10~ 4 candle/cm. a 

 1 lambert =l/ir candle/cm. a 



1 footlambert = 0.0003426 candle/cm.' 

 1 footlambert = l/ir candle/ft. 2 

 1 meterlambert = 1/w candle/m. 2 ) 



Consider an observed surface subjected to irradiation of one wave length so as to 

 produce a radiance having any desired level of luminance. Suppose the wave length of 

 the radiant flux emitted by the surface and the radiance of the surface be varied simul- 

 taneously in such a way as to maintain the luminance constantly at the given level. 

 Then there exists a particular wave length, X m , for which the radiance required to 

 yield this luminance is a minimum. 



Let N\ m = radiance of the surface at the wave length X m , 

 and N\ = radiance of the surface at the wave length X, 

 where the luminance is maintained constant at all wave lengths, then the ratio N\ n /N\ 

 is called the relative spectral luminosity for the given level of luminance. Its symbol is 

 yx = NxJNx. 



The term "luminosity" is recommended 4 in reference to the "capacities (reciprocals of 

 the required radiance) of radiant energy of the various wave lengths to evoke for a 

 particular observer visual sensation of equal brightness." 



Experimental procedures actually used to measure y\ are cited in the literature. 4, ' 



* Prepared especially for the Smithsonian Meteorological Tables, by L. P. Harrison, U.S. Weather 

 Bureau. 



1 Weaver, K. S., A provisional standard observer for low level photometry, Journ. Opt. Soc. Amer., 

 vol. 39, pp. 278-291, 1949. 



a Terminology is in conformity with that recommended by the Committee on Colorimetry of the 

 Optical Society of America. See references 3, 4, 5. 



•Committee on Colorimetry, Physical concepts: radiant energy and its measurement, Journ. Opt. 

 Soc. Amer., vol. 34, pp. 183-218, 1944. 



* Committee on Colorimetry, The psycho-physics of color, Journ. Opt. Soc. Amer., vol. 34, pp. 245- 

 266, 1944. 



8 Committee on Colorimetry, Quantitative data and methods for colorimetry, Journ. Opt. Soc. Amer., 

 vol. 34, pp. 633-688, 1944. 



8 Bur. Standards Sci. Papers 303 and 47S. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



