30 



HOW WE SEE 



factor affecting visual acuity is time: an 

 object is more visible the longer you look 

 at it. The data in Fig. 20 are taken from 

 an experiment by Ferree and Rand (24). 

 In this experiment, targets of various sizes, 

 illimiinated to various luminance levels, 

 were exposed for very short periods of time. 

 The observer's task was to identify the 

 target during the short exposure. The 

 data in Fig. 20 are plotted in terms of 

 speed, i.e., the reciprocal of time in seconds. 

 It is evident that when the subjects had 



240 



2 200 



120 



80 



40 



,10.2 FT-L 



I' Z 3 



VISUAL ANGLE (MiN.OFARC) 



Fig. 20. At any luminance level, less time is 

 required to see bigger objects. When size is held 

 constant, less time is required to see at higher 

 luminance levels. (After Ferree and Rand, 24) 



more time to look at the target, they could 

 see much smaller objects. Also evident is 

 the influence of the first factor — luminance. 

 At any exposure time, the subjects were 

 able to see smaller targets as the background 

 luminance increased. 



The range of background luminances 

 covered in this experiment is very small, 

 and there do not appear to be any more 

 complete studies of this function. There is 

 good reason to believe, however, that time 

 would become an especially critical factor at 

 luminance levels below the cone level. It 

 would be most instructive to have data for 



this function over a much greater range of 

 background luminances. 



It should be noted in passing that the 

 use of a "speed" scale in Fig. 20 is open to 

 the same objection that we had about the 

 visual acuity scale in Fig. 18. Speed, like 

 visual acuity, is a reciprocal measure which 

 distorts the original experimental measures. 

 Differences in the actual times represented 

 by speed values of 160 and 200 are so small, 

 0.006 as compared with 0.005 seconds, as 

 to be practically insignificant. Reconvert- 

 ing into time measures would not change the 

 essential correctness of our conclusions 

 about Fig. 20, but it would change our 

 estimates about the practical importance 

 of some of the trends shown here. 



Visual Acuity and Luminance Contrast. 

 The term luminance contrast may be 

 defined by the following equation : 



C. = 



Lo — Li 



l7~ 



Lo is the luminance of the object, Lb is the 

 luminance of the background on which the 

 object is located, and Cl is the luminance 

 contrast in percent. This term is most 

 frequently referred to as "brightness con- 

 trast," but the latter, in addition to being 

 technically incorrect, is confusing because 

 it has a special meaning for psychologists. 

 Psychologists think of brightness contrast 

 as the phenomenon of simultaneous bright- 

 ness contrast (see Appendix to this chapter). 

 Basic data showing the relationship 

 between visual acuity and luminance con- 

 trast are contained in Fig. 21. The two 

 parts of the figure were obtained from two 

 different studies, and, in general, they agree 

 fairly well for the lower contrast values but 

 do not agree for the high contrasts. These 

 discrepancies are due to differences in the 

 experimental conditions used in the two 

 studies. The data on the left were obtained 

 with long exposure times: the subjects were 

 allowed to look at the targets for three 

 seconds or more. The data on the right 



