54 



HOW WE SEE 



Dominant Wavelength. In Fig. 45 are plotted 

 the colors of a lemon (L) and ripe tomato (T). 

 A line has been drawn from the point W through 

 the color of the tomato, T, until it intersects the 

 spectrum line at approximately 596 m^. This 

 point of intersection with the spectrum line 

 defines the dominant wavelength of the color. 



.00 



radiant purity are fairly well correlated — the 

 greater the radiant purity of the stimulus, the 

 greater the colorimetric purity of the light. 



The Use of the ICI Diagram. A chromaticity 

 diagram is very useful because it tells the visual 

 scientist a lot about how the color looks. If 

 the color is far out to the right, it looks reddish. 



0.50 





 100 



0.50 



>- 

 o 

 a: 



UJ 



z 



UJ 

 UJ 



> 

 J 



Ui 



a: 





 100 



050 





 1.00 



0.50 







1,00 



0.50 



(B) 



1.00 



078 



too 



(D) 



0.0488 

 J, 



0.0309 



400 440 480 520 560 600 640 680 700 

 WAVELENGTH IN MILLIMICRONS 



Fig. 44. All five combinations of wavelengths shown here look exactly alike to the eye — very nearly 

 white. This illustrates the fact that the eye is an integrating mechanism rather than an analyzing one. 

 (After Moon, 68) 



Colorimetric Purity. The distance of T along 

 the line WS, expressed in percentage terms, 

 defines the colorimetric purity of the color. The 

 closer it is to ;S, the purer the color; the closer it 

 is to W, the more "washed out" it looks. This is 

 another way of saying that colorimetric purity is 

 correlated with the attribute of our sensations 

 called saturation. In general, colors with greater 

 amounts of colorimetric purity look more satu- 

 rated than colors with lesser amounts. As 

 another general rule, colorimetric purity and 



If the color is high on the chart, it looks greenish. 

 If the color is near the lower left-hand corner, 

 it does not contain much red or green and so looks 

 bluish. If a color is near the straight line running 

 from 380 m^ to 780 mfx, it will look purple. Purples 

 do not occur in the spectrum; they are actually 

 mi.xtures of red and blue. 



Another advantage of the chromaticity diagram 

 is that it immediately defines the mixtures which 

 are obtained from any combination of colors. 

 If 500 and 560 ni/u are mixed, for example, the 



