TABLES 58-77.— PHOTOMETRY 87 



Photometry is the measurement of light, and light has been defined by the 

 Illuminating Engineering Society as radiant energy evaluated according to its 

 capacity to produce visual sensations. 



TABLE 58.— THE EYE AS A MEASURING INSTRUMENT FOR RADIATION 



Part 1. — Theory 



As a measuring instrument for radiation, the eye is very selective, that is, it does not 

 respond equally to radiation of various wavelengths. The data in Part 2 give the relative 

 sensitivity of the eye to radiation of different wavelengths. Another peculiarity of the 

 eye is that its relative sensitivity changes with the intensity of the radiation that falls upon 

 it. This is shown by the data in Table 59. Also the absolute sensitivity of the eye varies 

 with the intensity of the radiation that falls upon it. This is shown by the data given in 

 Table 60. 



The data * on which Table 60 is based are not very extensive, but inasmuch as there is 

 now some active work on this subject by Lowry of the Eastman Kodak Co. there should 

 soon be available data for a wider range of field brightness. The data in Table 59 show that 

 the sensitivity of the eye to radiation of lower intensity increases faster toward the blue 

 end of the spectrum than in the red end. This is called the Purkinje effect. 



For light measurement at very low brightness care must be taken as to the standards 

 used. From the data given in Table 59 it can be shown that sources giving light of different 

 colors that were rated as equal by the average eye adapted to a field brightness of about 

 1 to 2 millilamberts would be rated quite differently for low field brightness, that is, for 

 the eye adapted to a field brightness of 10" 5 millilamberts. 



If the brightness given by two sources such as daylight and a carbon lamp be set equal 

 for a field brightness 1 to 2 millilamberts and then these brightnesses both reduced 

 mechanically to about 10" 5 millilamberts, the field of the daylight source would seem to be 

 about 2\ times as bright as that of the carbon lamp. 



SMITHSONIAN PHYSICAL TABLES 



