44 



HOW WE SEE 



light other than spatial and temporal inhomo- 



geneities. (18) 



The similarity between the definitions for light 

 and color means, in more familiar terms, that 

 white, gray, and black are colors. To distinguish 

 them from what most people mean by color, 

 the experiences of white, gray, and black are 

 sometimes called achromatic colors; the ordinary 

 colors like green, blue, and red are called chro- 

 matic colors. The characteristic thing about all 



Fig. 37. This illustrates the {jhenomciKni of 

 simultaneous brightness contrast. The two gray 

 areas are equal in reflectance. Do they look 

 equally bright? 



the.se definitions is that they involve sensations 

 and thus enter into the province of psychology. 



In most texts on vision and in the Illuminating 

 Engineering Nomenclature and Photometric 

 Standards approved in 1942 by the American 

 Standards Association (100), there are two sets of 

 terms and units to cover these two aspects of 

 visual stimuli : the purely physical aspect (radiant 

 energy), and the psychophysical aspect (radiant 

 energy as evaluated by a conscious human ob- 

 server, i.e., light and color). This is essentially 

 the distinction which was made and defined in 1922 



by the Committee on Colorimetry of the Optical 

 Society of America (89) and in 1931 by the Inter- 

 national Commission on Illumination. A con- 

 siderable amount of visual research since then has 

 shown that reports prepared by both groups 

 required modification, and in 1933, the Optical 

 Society of America appointed another Committee 

 on Colorimetry (hereafter referred to as the OSA 

 Committee) to consider revision and expansion 

 of these earlier reports. Some of the differences 

 of opinion among the various committee members 

 are summarized in an article by Jones (42). It 

 may be sufficient to point out that it required ten 

 years of work by this committee before unanimous 

 agreement could be reached. 



Sensation. The concepts of light and color as 

 defined above have been termed psychophysical 

 concepts by the OSA Committee (16) because they 

 cannot be identified simply with radiant energ}' 

 nor with visual sensation. Both are operationally 

 defined, i.e., the procedures used for evaluating 

 light and color constitute the definitions of these 

 concepts. But the OSA Committee recognized 

 what psychologists have long known: there are 

 differences between the magnitudes which measure 

 light and the magnitudes of our inner experiences, 

 i.e., sensations. The distinction can be shown 

 by a simple experiment: Cover one eye tightly 

 with a patch so that no light can reach it. Wear 

 this patch in ordinary illuminations for a half 

 hour. Now go into a dark room and look at a 

 very dim bulb, first with the eye that has been 

 covered, then with the other eye. The light 

 from the bulb is unchanged, A^et the visual sensa- 

 tions — the subjective impressions of brightness — 

 are considerably different to the two eyes. 



Another simple demonstration is the one shown 

 in Fig. 37. The gray areas in the centers of both 

 squares have the same amount of light coming 

 from them (assuming that the page is held so 

 that it is evenly illuminated), yet most people 

 will agree that the graj' area which is surrounded 

 by the black looks brighter than the other gray 

 area. Psychologists call this phenomenon simul- 

 taneous brightness contrast. Its importance 

 here is that it points up a fundamental difference 

 between amount of light, operationally defined, 

 and the magnitude of the resulting sensation. 

 The amounts of light from the two areas are the 

 same (as can be verified by looking at both gray 

 areas through holes in a piece of cardboard so 

 that only the gray areas are visible) but the 

 amounts of the sensations are different. 



Considerations such as these have forced the 

 OSA Committee to the position that the psycho- 

 physical concepts of light and color must be 

 difTerentiated from those which refer directly 

 to our sensations. Procedures for measuring the 



