18 



HOW WE SEE 



sures to seven different luminance levels — ■ 

 4,700, 2,090, 1,150, 447, 44, 20 and 4 mL— 

 for four minutes. He used two subjects, 

 but one of them had a pathological eye 

 condition and so cannot be considered 

 typical. The data for the normal subjeo-t, 

 Fig. 10, show how much more rapid dark 

 adaptation is when the pre-adapting light 

 has been dim. This is illustrated even 

 better by the data in Fig. 11, which shows the 

 time required for the dark adaptation 

 curve to reach a value roughly twice the 



25 



< ^ 



Q u. 



f 3 



to 



20 



15 



10 



• / 



ADAPTING LUMINANCE 

 (IN LOG MILLILAMBERTS) 



Fig. 11. The data of Fig. 10 have been replotted 

 here to show the time required for the dark adapta- 

 tion curve to reach a value near threshold as a 

 function of the pre-adapting luminance. 



threshold, (i.e., 4.30 nnL) as a function of 

 the pre-adapting intensity. It is evident 

 from Fig. 11 that near-threshold sensitivity 

 is reached in a matter of a few minutes even 

 if the pre-adapting light has been as high as 

 100 mL (log = 2). 



It is important to note, incidentally, that 

 the rod sections of the dark adaptation 

 curves in Fig. 10 do not have the same form. 

 When the luminance of the pre-adapting 

 light has been dim, e.g., 4 to 20 mL, the 

 dark adaptation curve drops much more 

 rapidly than it does following exposure to 



higher values, e.g., 2,090 and 4,700 mL. 

 We shall have something more to say about 

 this difference in speed of adaptation in 

 just a moment. 



Dark Adaptation as a Function of the 

 Duration of Pre- Adaptation. Another im- 

 portant determinant of the time required 

 for dark adaptation is the length of the 

 pre-exposure period. Data on this point, 

 also from the study by Haig (31), are shown 

 in Fig. 12. Only the rod portions of the 

 dark adaptation curves have been plotted. 

 In this part of his experiment, Haig used a 



10 20 



MINUTES IN THE DARK 



Fig. 12. Dark adaptation curves for one subject 

 following exposure to light of 447 mL for various 

 durations. Only the rod portions of the curves 

 are shown here. (Data from Haig, 31) 



single luminance, 447 mL, but varied the 

 duration of the pre-adapting period from 

 0.1 to 10 minutes. These data have also 

 been replotted to show how long adaptation 

 must continue to reach an arbitrary value 

 close to threshold (Fig. 13). Although the 

 data in Figs. 10 and 12 are for the same 

 subject, they were obtained on different 

 days, and the subject's sensitivity had 

 evidently changed somewhat in the mean- 

 time. For this reason, it has been necessary 

 to use a different criterion value in Fig. 13 

 from that used in Fig. 11. Nonetheless, the 

 story is clear. With fairly short durations 

 of pre-adaptation, dark adaptation is 

 virtually complete in a very few minutes. 



