10 



HOW WE SEE 



Each subject was required to vary the 

 intensity of the hght produced by radiation 

 of each of the wavelengths shown (412 m/z, 

 455 m^) -186 m/x, etc.) until it matched a 

 very dim white light very close to the 

 dark-adapted threshold of the eye. The 

 curve is called a relative luminosity curve 

 because it shows the relative luminosity 

 (or amount of light) produced by the various 

 wavelengths in an equal-energy spectrum. 

 Put in other words: if the completely 

 dark-adapted eye were to look at a very 

 dim, equal-energy spectrum, the different 

 parts of the spectrum would not appear to 

 be equally luminous. The most luminous 



scotopic curve is thus an expression of rod 

 sensitivity, the photopic curve an expression 

 of cone sensitivity. 



The Purkinje Shift. Although photopic 

 and scotopic relative luminosity curves 

 appear in almost all secondary literature on 

 vision, it is not generally understood that 

 these curves show the sensitivity of the eye 

 under extreme conditions. The photopic 

 curve is found when the eye is hght-adapted 

 to a fairly high luminosity level; the scotopic 

 curve results when the eye is completely 

 dark-adapted. When the luminance is de- 

 creased from photopic to scotopic levels, as 

 for example from afternoon illuminancse 



o 



UJ 



> 



< 



UJ 



'00 



75 



50 



25 



400 



500 



600 



700 



WAVELENGTH IN 111^ 



Fig. 5. Photopic and scotopic relative luminosity curves. The photopic data are from Gibson and 

 Tyndall (28); the scotopic data from Hecht and Williams (39). 



part of the spectrum occurs at 511 m/x; 

 462 and 555 m/i appear half as luminous 

 as 511 m/i, etc. 



The photopic relative luminosity curve 

 shown in Fig. 5 represents the average data 

 for 52 observers studied by Gibson and 

 Tyndall (28). Although their experiment 

 differs somewhat from the one of Hecht and 

 Williams in its details, the resulting curve 

 has essentially the same meaning with this 

 exception: the photopic relative luminosity 

 curve shows the relative luminosity of the 

 various wavelengths in an equal-energy 

 spectrum when the intensity of the spectrum 

 is well above the cone threshold. The 



through twihght to dusk, the transition 

 from cone to rod vision is not abrupt. The 

 eye does not start with photopic sensitivity 

 and switch suddenly to scotopic sensitivity. 

 Furthermore, the transition is gradual even 

 if the change in illuminance is abrupt. 

 Fig. 6 shows the results obtained by Walters 

 and Wright (95) on one of their two subjects. 

 The different curves were obtained at five 

 selected Imninance levels ranging from cone 

 levels to rod levels. Measurements were 

 made on a retinal area 10° from the fovea. 

 Notice the gradual shift in the luminosity 

 curves as the luminance changes from the 

 cone to the rod levels. Notice also that 



