TRANSMISSION CHARACTERISTICS 



117 



at a low level as in moonlight, the red field appears darker and the blue 

 lighter. This shift to rod vision accounts for the blue appearance of 

 objects as seen by moonlight. (Full moon 0.02 ft-canclle.) 



700 



« 600 



S.500 



I 400 



300 



■2 200 



<100 



Kb'nig's data 



Rod vision 



Low level 

 illumination 



Cone vision 



Daylight vision normal 

 observer 



4000 i 5000 1 +6000 



Violet Blue Green Yellow 



o 



Wavelengths in A 



Red 



7000 



Fig. Ill— 9. The visibility curves of the human eye or the brightness distribution 

 of the visible spectrum. Rod vision attains its maximum visibility at 5040 A; cone 

 vision, at 5560 A. The absolute visibility at maximum is 667 lumens per watt. 



The curves also show that the amount of radiant energy necessary 

 to produce a desired sensation of brightness varies enormously with the 

 wavelength. It is least in the yellow-green region at 5560 A, but for 



o 



low-level illumination this maximum brightness shifts to 5040 A. If a 

 low-intensity spectrum is viewed and decreased in brightness, a point 

 will be reached at which its violet and yellow ends disappear first, and 



Q 



the blue-green sensation due to 5040 A disappears last. 



In colorimetric methods of analysis as applied to clinical work, refer- 

 ence will again be made to this phenomenon in determining the choice 

 and relative accuracy of colorimetric comparison methods. 



Transmission Characteristics of the Ocular Media for Ultra- 

 violet Radiations 



Extensive experiments on animal eyes have shown that ultraviolet 

 radiation of high intensity and long exposure does not injure the retina 

 since the optical media involved are opaque to these short wavelengths, 

 but that this radiation will produce very painful conjunctivitis. 



o 



The cornea is opaque to all wavelengths below 2950 A, although some 

 evidence exists that the cornea of the rabbit is transparent to shorter 

 wavelengths. The cornea can transmit light of wavelength as small as 

 3000 A. 



