32 



HOW WE SEE 



to the much larger area around the visual 

 task. Actually, neither area has been 

 defined precisely in experimental work. 

 Nonetheless, it is generally agreed that 

 visual acuity in the center of the visual 

 field may be changed by varying the 

 luminance of the surrounds. The results 

 of a study by Lythgoe (62) on this function 

 are shown in Figs. 23 and 24. These 



for example, investigated the complete 

 visual acuity function for only three sur- 

 round luminances (Fig. 23), and his is 

 probably the most thorough study of these 

 functions. In addition, the data in Fig. 23 

 are for only two subjects; those in Fig. 24 

 for three subjects. Further, there are very 

 few observational points in either figure. 

 This is an especially critical matter when 



(00 



< 

 cr 



I- 

 -z. 

 o 

 o 



o 



EXPOSURE 

 TIME 



0.075 SEC. 

 ^0.300 SEC. 



0.5 0.7 



VISUAL ANGLE (MINUTES) 



Fig. 22. The interrelationships between visual acuity, background luminance, luminance contrast, 

 and duration of exposure. (Data from Cobb and Moss (15) as replotted by Luckiesh and Moss, 58) 



figures suggest that acuity is best when the 

 surround has about the same luminance as 

 the background immediately around the 

 visual task. As a practical illustration of 

 this principle, Hanes and Williams (34) 

 have shown recently that detectability of 

 targets on radar scopes actually improves 

 if the illumination in the radar room is kept a 

 little higher than is customary. 



Although these are informative studies, 

 they provide a good illustration of one of the 

 difficulties we mentioned earlier, viz., in- 

 completeness. The study by Lythgoe (62), 



one comes to draw a curve through the 

 observations. The curve drawn in Fig. 24 

 is mine, and it shows best acuity to occur 

 when the surround has a luminance of about 

 five equivalent foot-candles. When Lythgoe 

 drew a curve through the same data, he 

 made the curve reach its maximum much 

 further to the left, i.e., at a much lower 

 surround luminance. What one does with 

 data of this sort is somewhat arbitrary, 

 and a maximum could be precisely located 

 only if more data were available. Lythgoe, 

 finally, does not state what the exact size 



