DARK ADAPTATION AND NIGHT VISION 



13 



cones. If the relative luminosity curves are 

 replotted in terms of the amounts of energy- 

 involved, they assume the shapes shown in 

 Fig. 7. Although the curves in Fig. 7 were 

 actually computed from the data of Fig. 5, 

 Wald's data (91) show that this fomiulation 

 is in close agreement with the experimental 

 facts. This kind of a plot illustrates three 

 points clearlj^: (a) in progressing from cone 

 vision to rod vision, there is a shift in the 

 region of maximum sensitivity (from 555 m^ 

 to 511 m^t) — the Purkinje shift, (b) through- 

 out most of the spectrum the rods require 

 much less radiant energy for vision than 

 do the cones, and (c) the rods are about as 

 sensitive as the cones to radiation from the 

 long wavelength (red) end of the spectrum. 



Application to Low Luminance Photometry. 

 Having reviewed very briefly the behavior 

 of the eye at different luminance levels, we 

 are now in a better position to appreciate 

 why the problem of low liuninance photom- 

 etry is such a difficult one. The Purkinje 

 shift is at the root of the trouble, and the 

 situation is complicated by the fact that the 

 Purkinje shift is different for different retinal 

 positions. The development of standard 

 techniques and procedures to meet these 

 difficulties will require considerable work. 



Color Vision. The final point of interest 

 with regard to cone and rod function is that 

 color vision is possible only with cone vision. 

 When only rods are operating, it is possible 

 to distinguish between fight and dark colors 

 only in terms of the intensity of the reflected 

 or transmitted light. In rod vision, then, 

 all colors appear as a series of lighter or 

 darker grays. This does not mean, of 

 course, that color vision is impossible at 

 night. Common experience in observing 

 neon signs shows that colors can be seen 

 at night; but this is true only if they are 

 above cone thresholds. 



Dark Adaptation 



Going suddenly from a brightly lit 

 envirormient into darkness is a common 

 experience. Moviegoers endure it almost 



every time they attend the theater, and are 

 frequently embarrassed and annoyed by the 

 attendant consequences. Immediately after 

 being plunged into darkness, it is very 

 difficult to see things. Gradually, however, 

 more and more details of the environment 

 become perceptible, and, after a period of 

 from 10 to 30 minutes, very dim objects 

 can be seen. This increase in the sensitivity 

 of the ej^e is referred to as dark adaptation. 

 In this sense, it refers to a process — the 

 progressive increase in sensitivity of the 

 eye when the external illuminance has been 

 decreased. The term has also been used, 

 however, to refer to the constant state of 

 sensitivity reached by the eye when it has 

 remained in total darkness for some time. 



Instantaneous Threshold of the Eye After 

 Light Adaptation. Contrary to the impres- 

 sion one gets from reading war-time litera- 

 ture on dark adaptation, the eye is not 

 completely insensitive to all light imme- 

 diately after it has been plunged into 

 darkness. It can see objects which are 

 considerably dimmer than the general level 

 of luminances to which it had been adapted. 

 If this were not the case, of course, the eye 

 would not be able to see into deep shadows. 

 Instantaneous threshold measurements have 

 been made by Blanchard (3), and a portion 

 of his data are shown in Fig. 8. The 

 experiment was done by adapting the eye 

 to a large field illuminated to various levels. 

 At a signal, the adapting light was turned 

 off and the subject reported whether he 

 could see a much dimmer light which 

 replaced the adapting light. By many 

 repeated trials of this sort, it was possible 

 to find an instantaneous threshold lumi- 

 nance. Five different colors of adapting 

 and test lights were used: white, blue, green, 

 yellow and red. In any single trial, the 

 adapting and test lights were always of the 

 same color. 



Notice that there are two fairly distinct 

 portions to the three curves shown in 

 Fig. 8. The data for the different colored 

 lights are very much the same for adapting 



