188 HANDBOOK OF PHOTOGRAPHY 



from which it wUl be seen that the transition edges are not sharply defined. By mak- 

 ing a series of successive positive and negative prints of the original wedge spectro- 

 gram, using photographic materials and developers producing high contrast, it is 

 possible to sharpen this edge to obtain a negative or positive in which this edge is 

 rather clearly defined. This procedure is seldom used, however, for it is customary 

 to inspect the results directly from the original wedge spectrogram, determining from 

 this visual inspection the relative sensitivity at various wavelengths. In the original 

 wedge spectrogram, the relative sensitivity will be indicated by the amount of silver 

 deposit and the extent of blackening of the negative; the examples shown are prints 

 made from original wedge-spectrogram negatives. 



It should be noted that the results obtained in this case represent, not only the 

 spectral-sensitivity characteristics of the photographic material, but the combined 

 results of the photographic sensitivity and the spectral-energy distribution of the light 

 source. Thus, the wedge spectrogram represents the spectral photicity of the mate- 

 rial for the given light source and provides curves which are at variance with those 

 which may be obtained by methods using monochromatic sensitometers. 



Typical wedge spectrogram for panchromatic material. 



The wedge used over the slit in Fig. 22 has a linear and constant density gradient, 

 and therefore the distribution of radiation along the slit increases logarithmically 

 from one end of the slit to the other. The resultant envelope curves are thus logarith- 

 mic in form and cannot be compared directly with the curves obtained by mono- 

 chromatic sensitometric methods. It should also be kept in mind that, if the wedge 

 is not completely a neutral gray over the range of wavelengths for which the light 

 source produces radiations and the photographic material is sensitive, then the over- 

 all wedge spectrograms will be affected by the selective absorption of the wedge. For 

 wavelengths below about 450 mju wedges have appreciable selective absorption, so that 

 the apparent decrease in sensitivity of photographic materials at the short visible 

 wavelengths is partly due to selective absorption of the wedge. 



Stepped wedges, rather than constant-gradient wedges, have also been used and 

 have been found to be useful where it is desired to make density measurements directly 

 from such spectrograms. If the wedge is removed completely, the density of the 

 silver deposit will give an indication of the spectral sensitivity of the photographic 

 material when used with the given light source. In this case, measurements of the 

 amount of silver deposit at various wavelengths by means of a microdensitometer 

 enable spectral-photicity curves to be obtained. 



Selective Absorption Methods. — The spectral sensitivity of a photographic material 

 as determined by the methods of monochromatic sensitometry and by the usual 

 spectrographic technique is most conveniently and almost necessarily expressed 

 graphically, the usual mode being a curve showing sensitivity as a function of wave 

 length. It is almost impossible to express the information relative to the spectral 

 sensitivity as derived by these methods in brief numerical terms. It is frequently 

 desirable to sacrifice some of the precision of the methods already discussed for less 



