DARK ADAPTATION AND NIGHT VISION 



11 



there is an increase in the relative luminosity 

 of blue and blue-green lights (wavelengths 

 below approximately 510 m/z) as adapta- 

 tion proceeds from cone to rod levels. 

 This shift in the relative luminosity curve is 

 variously called the Purkinje phenomenon, 

 Purkinje effect, or Purkinje shift, for the 

 famous Bohemian physiologist who dis- 

 covered it. 



are used to provide cone levels of illuminance 

 on a working area, the eye of an observer 

 nearby will have sensitivity values as 

 illustrated by the photopic relative lumi- 

 nosity curve. As the observer moves away 

 from the light sources, the illuminance 

 falling on his eye will decrease according 

 to the inverse square law, and the sensitivity 

 of his eye will gradually undergo the 



100 



80 



CO 



O 



E 60 



3 



> 



< 



_J 



20 



400 



500 



600 



700 



WAVE-LENGTH IN ttl^ 



Fig. 6. Relative luminosity curves at five luminance levels (1 is the lowest luminance; 5 the highest). 

 The gradual shift in the luminosity curve is called the Purkinje shift. (Data from Walters and Wright, 

 95). 



Practical Import of the Purkinje Shift. 

 The practical significance of the Purkinje 

 shift is apparent. If variously colored 

 lights, all of equal size and intensity, are 

 used to provide cone levels of illuminance at 

 night, these lights will not appear to be 

 equally luminous to a distant observer. 

 Green, blue-green and blue lights "outlive" 

 other colors in visibility as the distance 

 between the light source and observer 

 increases. The reason is now easy to 

 understand. If the variously colored lights 



Purkinje shift, i.e., will become relatively 

 more sensitive to the shorter wavelengths. 

 Thus, although the lights may all have 

 appeared equally intense at short distances, 

 the green, blue-green, and blue ones will be 

 visible for much greater distances. This, 

 however, is true only if the distances 

 involved are much greater than the largest 

 dimension of the light source. 



The Purkinje Shift in Different Retinal 

 Locations. Since there are no rods in the 

 fovea, it might be anticipated that the. 



