12-7] IMPORTANT CHARACTERISTICS OF THE HUMAN OPERATOR 639 

 Fig. 12-20 Table of Approximate Brightness. 



100,000 mL 

 10,000 mL 



1,000 mL 



100 mL 

 10 mL 



1 mL 



0.1 mL 

 0.01 mL 



Upper limit of visual tolerance 



New snow on clear day or upper surface ot clouds at noon 

 Average sky on hazy day at noon 

 Average sky on clear day 

 Average earth on clear day 

 Average sky on cloudy day 



Average earth on cloudy day 



White page in good reading light 



White paper 1 tt from a candle 



Average chart brightness at lowest readable brightness level 

 Snow in full moon 



is placed alongside a color whose wavelength is 60L75 mn, a difference in 

 hue will be discerned. 



The eye responds differentially to radiant energy; i.e., the energy 

 required to elicit an equal brightness response varies with wavelength. To 

 respond differentially does not necessarily imply color discrimination, but 

 merely that the response to different spot colors requires more or less 

 energy. Fig. 12-19 is called the visibility curve and shows the relative 



