DARK ADAPTATION AND NIGHT VISION 



21 



may be very brief. The red goggles serve 

 two functions: (a) They reduce the overall 

 amount of light reaching the eye so that the 

 eye is light-adapted to a lower luminance 

 level, (b) They admit only red light to the 

 eye, with the result that subsequent dark 

 adaptation is more rapid. 



During the war, a great amount of 

 research time was spent on the evaluation of 

 different red filters in terms of their efficiency 

 for dark-adapting the eye. In a general 

 kind of way, it was appreciated that a 

 filter w^as better for dark adaptation if it 

 transmitted fewer of the shorter wave- 

 lengths. As a rule, however, filters which 

 transmit less short-wave radiation are also 

 denser, i.e., they transmit less light generally. 

 Aside from these crude generalizations, 

 there was no principle that the visual 

 scientist could use in evaluating the efficiency 

 of red goggles, and everj^ time the problem 

 arose, different filters had to be compared 

 with each other in a new experiment. 



The writer believes that a more universal 

 generalization is possible. The germ of the 

 idea comes from an article by Kohlrausch 

 (49) in which he evaluated various filters 

 in terms of what he called the D/T ratio 

 (Dammerungswert/Tageswert = twilight 

 value/daylight value). The same idea is 

 also used in the article by Hecht and Hsia 

 (37). Briefly, the procedure is this: the 

 transmission of the filter in question is 

 multiplied by the relative scotopic lumi- 

 nosity curve. The ratio of the area under 

 the resultant curve to the area under the 

 scotopic luminosity curve yields the scotopic 

 value (S). A similar computation with the 

 photopic luminosity curve yields a photopic 

 value (P). The S/P ratio should be a 

 measure of the relative efficiency of the 

 filter for dark adaptation. The higher the 

 ratio the worse the filter; the lower the 

 ratio, the better the filter. The validity of 

 this proposal has never been thoroughly 

 tested, and it is probably worth investiga- 

 tion. If the principle is valid, it should be 

 possible to work out a table of values 



showing instantaneous thresholds for the 

 average eye following light adaptation with 

 filters and lights (or lights alone) of various 

 S/P ratios and at various luminance levels. 

 Not only would this eliminate the tedious 

 testing of pairs of filters every time some 

 manufacturing concern proposed a new 

 filter, but it would also provide a rational 

 way of specifying the kind of filter and 

 amount of light (or light alone) which should 

 be used for purposes of dark adaptation. 



Research on Methods of Accelerating Dark 

 Adaptation. The flexibility of troops en- 

 gaged in night maneuvers has always been 

 hampered by the slowness of the dark 

 adaptation process, because military opera- 

 tions do not always occur wdth enough 

 advance warning to give men time to adapt 

 their eyes. It would obviously be of great 

 tactical value to have some method of 

 accelerating the dark adaptation process. 



In recent years a number of articles have 

 appeared from Russian laboratories claiming 

 to show enormous effects of intersensory 

 stimulation on dark adaptation and night 

 vision. Kekcheyev (46), for example, re- 

 ports, "Several months ago we experimented 

 in expediting adaptation by means of light 

 muscular exercise. . . . Experiments made on 

 ten subjects with the help of the adaptom- 

 eter revealed that it was possible in this 

 way to reduce the period of adaptation from 

 25-45 minutes to 5-6 minutes." In another 

 article (45), the same author claims that 

 "In some instances the period of adaptation 

 dropped from forty-five minutes to eight 

 minutes" with combinations of tastes. 

 Kekcheyev then goes on to say in the same 

 article that auditory, olfactory, gustatory, 

 labyrinthine, thermal, pain, tactile, pro- 

 prioceptive, and interoceptive stimuli pro- 

 duce changes in the sensitivity of the dark- 

 adapted eye. This is summarized b}^ his 

 statement, "It can be seen, therefore, that 

 excitation of any receptor produces changes 

 in the sensitivity of the dark-adapted eye" 

 (45). Other researches show that the 

 "sensitizing" stimulus need not be sensed 



