PHOTOGRAPHIC SENSITOMETRY 



187 



etric strip exposed to white light a gamma approximately equal to that at which the 

 material is usually developed in practice. In Fig. 20 is shown a spectral-sensitivity 

 curve determined in this manner. This is for high-speed panchromatic motion- 

 picture film, the development time used being that which gives a gamma of 0.7 on a 

 white-light sensitometric strip. 



"It should be borne in mind that the spectral sensitivity curve, when plotted in 

 accordance with the specifications given in this section, represents the characteristics 

 of the photographic material itself, quite apart from any consideration of the energy 

 distribution in the light source used."'^ The curve of Fig. 20 shows the spectral 

 sensitivity when used with a hypothetical light source emitting equal amounts of 

 energy at all wavelengths, and corresponding, therefore, to ideal white light. If it is 

 desired to determine the effective spectral response when used with some other light 

 source in which the spectral-energy distribution deviated from this equal-energy 

 distribution, it will be necessary to compute a new family of characteristic curves 

 showing the effective spectral sensitivity of the photographic material when used with 

 the desired light source. These effective spectral sensitivity curves may be computed 



'ffrachbr 



Neuiral 



wedge^ ShuHer 



/Condenser 

 ' iSource 



Fig. 22. — Optical system for wedge spectrograph for making determinations of spectral 

 sensitivity of films when used with specified light source. This method does not determine 

 absolute film characteristics, but relates them to the light source employed in making the 

 measurements. 



from the spectral sensitivity for curves of an ideal white light source and the curve 

 of spectral-energy distribution of the light source actually used. The method of 

 determining spectral sensitivity by means of monochromatic sensitometers, although 

 precise, is tedious and time consuming. 



Dispersion Methods Using Wedges with Spectroscopes. — More rapid, although less 

 accurate, methods of determining the relative spectral response characteristics of 

 photographic materials have been devised in which a spectrum, such as that from a 

 prism or grating, is permitted to fall on a film after having passed through a neutral 

 density wedge of constant gradient. The wedge is so placed with respect to the 

 spectrum that it attenuates all portions of the spectrum equally well. This is accom- 

 plished by spreading out the spectrum in one direction and by placing the wedge so 

 that its density contours are perpendicular to the wavelength scale. Figure 22 

 shows the essential elements in such a wedge spectrogram method of determining the 

 spectral characteristics of emulsions. 



The wedge spectrogram method has the advantage of speed and simplicity 

 and gives in graphic form good indications of the relative spectral sensitivity of 

 the photographic material. A typical wedge spectrogram is shown in Fig. 23, 



' Jones, L. A., "Photographic Seiisitometry," Eastman Kodak Co. (1934). 



