2 : 4/ Light and the Eye 



47 



say more than 50 milliseconds, only the intensity is important. The 

 exact size of the test patch used becomes very important if it is two 

 minutes of arc or less. With very small test patches, the exact location 

 of the test patch very markedly affects the shape of the relative luminosity 

 versus wavelength curve. For larger test patches, threshold curves are 

 obtained which do not depend specifically on the particular area on the 

 retina which is illuminated. 



The general shape of the relative luminosity curves for photopic and 

 scotopic vision is shown in Figure 12. Owing to the definition of 



1.0 



0.8 



^0.6 



o 



c 



6 



3 



0.4 



0.2 



V \ f \ 



400 500 600 



Wavelength (m\t) 



700 



Figure 12. Relative luminosity curves. The curve for the 

 dark-adapted eye is labeled b and for the light-adapted eye, a. 

 After Committee on Colorimetry, The Optical Society of 

 America, The Science of Color (New York: Thomas Y. Crowell 

 Company, 1953), p. 225. 



relative luminosity, the absolute height of the curves does not have any 

 significance. Much greater intensities are needed for photopic vision 

 than for scotopic vision. (This difference is part of everyday experience. 

 After the lights are turned off at night a room looks totally dark, but 

 gradually one can see more and more objects in it.) Luminosity thresh- 

 old measurements are not easy to perform. More than half an hour is 

 necessary for dark adaptation. Care must be taken to illuminate the 

 same area of the retina, and many other precautions must be observed 

 as well. However, the relative luminosity curves do lead to reproducible 

 results. 



The separation of the maximum points of the scotopic and the photopic 



