186 



HANDBOOK OF PHOTOGRAPHY 



but the results in this case will not be directly comparable with those determined in 

 the first instance. 



For theoretical purposes there is considerable argument for defining spectral 

 sensitivity in terms of the energy required to give a densitj^ of unity when development 



1.6 



1.4 



^ 1.2 



1^ 1.0 



E 



(B as 



0.4 



Q2 







300 400 500 600 700 



Fig. 20. — Typical curves showing the variation of y with the wavelength of light, for the 

 development times indicated. The wavelengths of light are measured in millimicrons. 



2.8 

 Z4 

 20 



_^ 1.6 



"w 



c 



0.8 



0.4 







-2.0 -1.4 -1.8 0.2 0.6 04 08 

 Log 10 E (ergs/cm 2; 



Fig. 21. — Z)-logio E characteristics of typical negative material, measured at four different 

 wavelengths, for identical processing conditions. In general the y increases with the wave- 

 length, and this fact has important practical considerations in color photography. 



2.0 



for all wavelengths is carried out to a gamma of unity. For practical purposes, 

 however, it seems that the evaluation of spectral sensitivity in terms of a fixed develop- 

 ment time is more suitable, and, in order to discount somewhat the misleading effects 

 of gamma variation, it seems probable that the determination of the energy per unit 

 area of the photosensitive material required to give a density of unity, for a fixed time 

 of development, is most satisfactory as a mode of expressing spectral sensitivity. 

 The most suitable development time is probably that which produces on asensitom- 



