VISUAL ACUITY 



31 



were obtained with brief exposures: 0.17 

 seconds. Since we have already seen that 

 visual acuity is a function of time, it is not 

 surprising to find that the two sets of data 

 do not agree. 



Interrelationships Between Factors. Each 

 of these factors in visual acuity is related to 

 every other one. A reduction in an}^ one 

 factor — background luminance, size, con- 

 trast, or time — may be compensated for by 

 an increase in one or more of the others. 



in Fig. 22. The area above the three- 

 dimensional curve shows those objects 

 which can be seen clearly; the area below 

 the curve represents those objects which 

 cannot be seen. This plot was derived 

 from over 100,000 separate measurements, 

 and it forms the basis for many of our 

 predictions about visual acuity under many 

 practical conditions. This set of data 

 does not cover the full gamut of luminance 

 values, and it has been worked out for only 



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O.OOOI 



OOCI 



LUMINANCE OF TASK BACKGROUND IN FOOTLAMBERTS 



Fig. 21. Visual acuity as a function of background luminance and the luminance contrast between 

 the object and its background. The data on the left are from Connor and Ganoung (20); those on the 

 right from Cobb and Moss (15). 



For example, an object which is so small 

 that it is just below threshold may be made 

 visible by increasing the background lumi- 

 nance, time of exposure, or luminance 

 contrast. This is true within certain limits, 

 of course, because it is possible to find an 

 object so small that it cannot be seen by the 

 unaided eye under any circumstances. 



The interrelationships between these fac- 

 tors have been investigated by Cobb and 

 Moss (15) and have been summarized in 

 graphical form by Luckiesh and Moss (58) 



two exposure times. However, it is the 

 best set of data we have on this problem, 

 and the more complete investigation of the 

 interrelationships of these factors must 

 wait upon further research. 



Luminance of Surround. According to 

 many illuminating engineers, another im- 

 portant factor influencing visual acuity is 

 the luminance of the large area surrounding 

 the visual task. The visual task usually 

 refers to a small area (5° or less) in the 

 center of the field of view and the surround 



