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HANDBOOK OF PHOTOGRAPHY 



spectrum for which the film is relatively insensitive. A 15-watt red lamp used as a 

 safe light for noncolor-sensitive emulsions may produce no appreciable exposure 

 or fog, whereas a 15-watt blue lamp might badly expose the film under the same 

 conditions. 



180 



160 



140 



c?l20 



(U 



iSioo 



^ 80 

 ,2 60 



40 



20 



300 



600 



700 



400 500 



Wavelength in m/u. 



Fig. 1. — Curves of energy output plotted against wavelength for tungsten lamps operating 

 at three different temperatures. 



The sensitivity of the human eye to equally intense radiations of different wave- 

 lengths is termed the visibility curve. The visibility curve for the average human eye 

 is given in Fig. 2. The net over-all response of the human eye to a light source con- 



400 



700 



Fig. 2.- 



500 600 



Wavelength in Millimicrons 



-Spectral sensitivity of the normal human eye. 



taining energj^ at many wavelengths depends upon the visibility curve of the human 

 eye, as well as spectral-energy distribution of the light source, and is termed the 

 luminosity. If a light of given intensity is yellow, the luminosity will be high, as 

 the human eye is most sensitive to yellow; if the same amount of energy is used in a 



