7:1/ Neural Aspects of Vision 



121 



so that by choosing the proper amounts of these, one could match the 

 chromaticity of a given light in terms of the sensation it evoked in the 

 average observer. If the amounts of each of the three standards are 

 indicated by x, y, and z respectively, then one may represent symboli- 

 cally a light A, by 



A = x A + y A + z A 

 and a light B, by 



B = x B + y B + z B 



If one now adds equal amounts of A and B to form a new light C, which 

 may be represented as 



C = x c + y c + z c 



then it is found that 



x A + x B = x c y A + y B = y c and z A + z B = z c 



In general, any algebraic combination pf colored lights is matched by the 

 corresponding algebraic combination of the amounts of the standards 

 matching these lights. 



4> 



400 500 600 700 

 Wavelength (m|x) 





400 500 600 700 

 Wavelength (mnJ 



"3 



400 500 600 700 

 Wavelength [myt) 



Figure I. Standard CLE. tristimulus values of unit energy 

 for indicated wavelengths. After Committee on Colorimetry, 

 The Optical Society of America, The Science of Color (New 

 York: Thomas Y. Crowell Company, 1953) pp. 242-243. 



In order to standardize the description of chromaticity, the Inter- 

 national Congress on Illumination agreed on three artificial standards. 

 These were chosen so that a monochromatic light at any wavelength in 

 the visible spectrum is matched by an average observer by the amounts 

 •*/b y~\-> i>\ shown in Figure 1. The curve y K has the same shape as the 

 average photopic luminosity curve; it gives the luminosity of a given 

 light. The curves were normalized so that a white light (of equal 



