VISUAL ACUITY 



29 



in visual acuity from 2.0 to 2.5, on the 

 other hand, represents a change in object 

 size of only 0,1 minute. In terms of the 

 actual size of the object to be seen, then, 

 the visual acuity scale greatly expands 

 differences between the smaller objects and 

 contracts differences between the larger ones. 

 The curve in Fig. 19 shows the same 

 data as those in Fig. 18. In Fig. 19, 

 however, we have plotted the actual size 

 of the smallest detail which can be seen. 

 Although the two curves plot the same 



10 



likely that the kind of plot illustrated in 

 Fig. 19 is more meaningful than the kind 

 illustrated in Fig. 18. 



Visual Acuity at Night. These data are 

 also important for telling us about seeing 

 at low luminance levels. The decrease in 

 acuity at night means that identification of 

 objects must depend on the perception of 

 generalized contours and outlines and not 

 of small distinguishing features. Wires, 

 picket fences, and telephone poles may be 

 invisible a few hundred feet away at night. 



± 3 



UJ 



< 



< 

 to 



0.7 



05 

 04 





BACKGROUND LUMINANCE IN LOG MILLILAMBERTS 



Fig. 19. This is the same curve as in Fig. 18, except that the ordinate is now a logarithmic plot of the 

 smallest detail visible. Notice that this curve shows lesser gains as luminance is increased at the 

 higher values. 



data, what they show is quite different. 

 The curve in Fig. 18 appears to show^ large 

 increases in visual acuity when the back- 

 ground luminance is increased from 10 to 

 1000 (log = 1 to log = 3) mL. Fig. 19, 

 on the other hand, shows that in terms of 

 the smallest detail which can be seen, 

 there is not much to be gained by increasing 

 the luminance within this range. It is 

 important to note, incidentally, that the 

 ordinate in Fig. 19 is logarithmic. Thus, 

 even in this figure the high values are 

 compressed and the low ones expanded. 

 For most applied visual problems, it is 



Aircraft and ships are least visible when 

 viewed from dead astern, because their 

 areas are smallest in that direction. For 

 this reason, night interception tactics during 

 the war required that enemy aircraft be 

 followed from rear-above or rear-below, 

 rather than rear-level. Similarly, small 

 terrain features, a small building, smoke- 

 stack, or a bridge, may not be visible from 

 the air at night, and recognition must 

 depend on rather large ground features — 

 surf and sand, large clumps of trees, rivers, 

 lakes, and large concrete installations. 

 Time and Visual Acuity. The second 



