DAEK ADAPTATION AND NIGHT VISION 



23 



expect that the combat soldier or sailor 

 will be at the peak of his seeing ability and 

 that there is little else one can do to help 

 him. 



Night Vision 



As we have seen above, dark adaptation is 

 usually measured by determining the small- 

 est amount of light an individual can see at 

 various times after he has gone into a 

 dark room. Actually, however, the fighting 

 man is seldom concerned with the mere 

 perception of light, but rather with the 

 recognition and identification of forms. 

 During the war most people assumed that 

 these functions were synonymous and, in 

 fact, this idea is still common even among 

 visual scientists. One important conse- 

 quence of this belief was that there was a 

 fair amount of muddled thinking about 

 night vision tests. Tests of dark adapta- 

 tion, contrast sensitivity, form discrimina- 

 tion at low luminances, glare recovery, and 

 many others, were proposed as tests of night 

 vision, and it was generally assumed that 

 these tests all measured the same thing. 

 More recent research has shown that this is 

 not the case, and there has now been 

 accumulated a fair amount of evidence to 

 show that some of the principles of night 

 vision may be different from those of dark 

 adaptation. It is worth while to review 

 briefly some of these lines of evidence. 



Factors in Night Vision. For purposes 

 of this section, night vision will be defined 

 very broadly as the ability to see at rod 

 levels of luminance. A few authorities in 

 the field have generally recognized that, 

 while dark adaptation may set the limit, 

 other factors play an important role in 

 night vision. Thus, Livingstone (52) states 

 that "The factors involved in the awareness 

 of the presence of an object and in the acute 

 analysis of the stimulus producing that 

 awareness consist, it seems, of a highly 

 complicated inter-reaction between physi- 

 ology and psychology." In one of the few 

 experimental studies in this field, Craik and 

 Vernon (21, p. 229) conclude: 



The ability of the . . . subjects to perform 

 simple perceptual tasks during dark adaptation, 

 such as reading the position of a dial hand, was 

 very closely related to their absolute thresholds 

 of vision. But the perception of more com- 

 plicated material, such as silhouette pictures, 

 was determined only partially by the absolute 

 threshold in cone vision; and still less by the 

 absolute threshold in rod vision. The ability 

 of the subject to interpret the vague sensory 

 data obtained in rod vision, and hence to make 

 out the meaning of the picture, was affected 

 by a variety of purely psychological qualities. 

 Intelligence, education, and familiarity with 

 and experience of the particular type of mate- 

 rial and situation, were of great assistance. 

 Affectively based attitudes were also important. 

 Too much caution or timidity prevented the 

 necessary amount of guessing at the meaning 

 of the pictures; too much recklessness, and 

 suggestibility to irrelevant influences, produced 

 inaccurate guessing. 



About as much as we can say with 

 authority about these other factors is that 

 it is almost certain that perceptual and 

 mental factors contribute to night vision, 

 but that the exact amount of their influence 

 cannot be assessed with our present 

 knowledge. 



Data of Night Vision Tests. Data from a 

 large number of scattered wartime reports 

 show that measures of dark adaptation do 

 not correlate with scores on tests involving 

 visual acuity or form perception at rod 

 levels of luminance. Hammond and Lee 

 (32), for example, found no correlation 

 between measures of dark adaptation deter- 

 mined with the Hecht-Shlaer Adaptometer 

 and any of their other tests of night vision. 

 Rowland and Rowland (78) measured three 

 different aspects of night vision: (a) the 

 light threshold, (b) the minimum intensity 

 of light required for the subject to locate a 

 test object, and (c) the minimum intensity 

 required for form discrimination. The cor- 

 relations between light thresholds and the 

 other two measures were hardly significant: 

 0.29 and 0.22. The correlation between 

 the latter two measures, which required the 

 observer to locate or identify a form, or 

 both, was much higher: 0.63. Keil (43), 



