DARK ADAPTATION AND NIGHT VISION 



19 



Different Rates of Dark Adaptation. It is 

 also clear from Fig. 12 that the rate of dark 

 adaptation following a very short pre- 

 exposure period is more rapid than the rate 

 following long pre-exposures. To get better 

 evidence for this, Haig calculated a crude 

 measure of the slope of each dark adaptation 

 curve. He defined the slope as the recip- 

 rocal of the time required for the curve to 

 drop 0.4 log units and computed it for two 

 contiguous 0.4 log unit intervals, from 

 1.19 to 0.79 and from 0.79 to 0.39 log units 

 above the threshold. All that is really 

 important here, however, is that the slope 



5i 



^ '^ 

 X o 



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5 < 



15 



10 





1 



MINUTES OF LIGHT 

 ADAPTATION 



Fig. 13. The data of Fig. 12 have been replotted 

 here to show the time required for dark adaptation 

 to reach a value near threshold as a function of 

 the duration of pre-adaptation. 



tells us how fast the dark adaptation curve 

 dropped. The higher the slope, the more 

 rapidly the curve dropped; the lower the 

 slope, the more slowly it dropped. 



Slope values, plotted as functions of the 

 luminance of the pre-adapting light and 

 duration of pre-exposure, are shown in 

 Fig. 14. This figure shows very clearly 

 that dark adaptation may progress at 

 different rates. It is much faster (a) when 

 the pre-adapting intensity has been 44 mL 

 or less, or (b) when the pre-exposure has 

 lasted less than one minute. If the pre- 

 exposure intensity has been higher than 

 44 mL or has lasted longer than one minute, 

 dark adaptation proceeds at much slower 



rates. Haig (31) and Wald and Clark (92) 

 have provided an explanation for this 

 phenomenon in terms of two different kinds 

 of photochemical reactions known to occur in 

 the retina. We need not be concerned here 

 with the details of these photochemical 

 processes and the point is mentioned only to 

 show that the entire story makes sense. 



The importance of this finding should be 

 obvious. Whenever possible, the specifica- 

 tions for illuminance in ready-rooms should 

 be such that they will permit the eye to 

 dark-adapt at fast rates. Another implica- 

 tion, not so obvious perhaps, is that exposure 



• 



ADAPTING LUMINANCE 

 ( IN LOG M1LLILAM8ERTSI 



MINUTES OF LIGHT 

 ADAPTATION 



Fig. 14. The slope of the dark adaptation curve 

 as a function of pre-adaptation luminance (left) 

 and duration of pre-adaptation (right). These 

 curves were computed from the data shown in 

 Figs. 10 and 12. See the text for a fuller ex- 

 planation. 



of the eye to a dim light for a short period 

 of time does not completely destroy all dark 

 adaptation. It is important to remember 

 that Haig's data were obtained with a 

 luminance of 447 mL, which is a very high 

 value. Fragmentary data from a study by 

 Peckham (73) show that if the eye has been 

 dark-adapted and is exposed to a luminance 

 of 7.1 mL for eight seconds it returns to its 

 fully dark-adapted condition within a min- 

 ute. This means that no really great harm 

 is done if a soldier or sailor has to turn on a 

 very weak light for short periods of time to 

 examine a map or message. 



Further Research Needed. Although the 



