DARK ADAPTATION AND NIGHT VISION 



17 



ied the visual sensitivity of a subject who 

 remained in darkness for six hours. He 

 found that the curve of sensitivity changed, 

 although very slowly, throughout the six- 

 hour period. Anj^ statement about the 

 time required for adaptation, therefore, 

 must use some arbitrary criterion of "com- 

 plete adaptation." Perhaps some definition 

 such as this would suffice: for all practical 

 purposes, the eye is dark-adapted when the 

 mean threshold value does not change by 

 more than 0.1 log units in two consecutive 

 five-minute intervals. As far as the writer 

 is aware, no such standard criterion has 

 ever been used in dark adaptation studies. 



Dark Adaptation as a Function of Pre- 

 Adaptation. The second reason we must be 

 cautious about stating the length of time 

 required for complete dark adaptation is 

 that the course of dark adaptation is 

 dependent on the amount of previous light 

 adaptation. The data in Fig. 9 were 

 obtained by having the subjects fixate a 

 large surface with a liuninance of 1,100 mL 

 for three minutes before dark adaptation 

 was started. This exposure to light before 

 dark adaptation begins is called light 

 adaptation, or pre-adaptation. A pre- 

 adaptation period is used routinely in 

 laboratory studies of dark adaptation to 

 insure that subjects start with a constant 

 amount of photochemicals bleached in the 

 retinae of their eyes. It is a control used by 

 the laboratory scientist to obtain more 

 uniform and comparable dark adaptation 

 records from different individuals. Since 

 one subject may report to the laboratory 

 after having been out in sunlight (with most 

 of the photochemicals in his eye bleached 

 by the strong light), while another may 

 come in after an hour's work in a dim 

 laboratory (with a relatively small amount 

 of photochemicals bleached), the easiest way 

 to start all subjects at the same level is to 

 bleach their retinae the same amount by 

 exposing them all to a very intense light. 

 As a general rule, then, the pre-adapting 

 lights used in laboratory experiments are 



very intense. The luminance of the pre- 

 adapting light used by Sloan in her study is 

 plotted in Fig. 9 to show how high it is. 



As a practical matter, however, men who 

 are required to work at night are very 

 seldom exposed to luminances as high as 

 those used for pre-adaptation in the labora- 

 tory. Pilots sitting around a ready-room 



iO 20 30 



MINUTES IN THE DARK 



Fig. 10. Dark adaptation curves for one subject 

 following exposures to lights of various luminances 

 for four minutes. The solid symbols indicate that 

 the color of the test light (violet) could be identi- 

 fied at threshold; open s>Tnbols indicate that the 

 color of the test light could not be recognized. 

 The test light appeared colorless at threshold 

 following pre-adaptation to the three lowest 

 luminances. (Data from Haig, 31) 



at night, for example, would probably be 

 adapted to luminances of 1 to 30 mL even if 

 the ready-room were illuminated with several 

 oQ-watt hulhs. The point is frequently 

 overlooked that the time required for dark 

 adaptation decreases rapidly as the lumi- 

 nance of the pre-adapting light decreases. 

 Data on this point come from a study by 

 Haig (31). This investigator studied the 

 course of dark adaptation following expo- 



