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HANDBOOK OF PHOTOGRAPHY 



Filter Factor. — The filter factor is not an inherent property of the filter alone which 

 can be specified once and for all. Instead, the filter factor depends upon its spectral- 

 transmission characteristics, the spectral characteristics of the source of radiant 

 energy with which the filter is to be used, and also upon the spectral sensitiveness of 

 the film upon which the modified light acts. Of these factors the only one which is 

 inherently a property of the filter itself is spectral transmission of the filter. As the 

 two other factors vary, the filter factor will change. 



Graphical Construction Showing Action of Filters. — Before interpreting the theory 

 of filters, it will be well to consider some fundamental principles relating to radiation, 

 filter transmission, and the spectral sensitivity of the photosensitive material. 



100 



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100 



100 







100 



350 400 450 500 550 600 650 700 750 

 Wavelength in Millimicrons 



Fig. 5. — Spectral curves of sunlight (top), a yellow filter (second) and panchromatic 

 material (third). The effect of exposing the panchromatic film to light of daylight quality 

 is shown in curve A (bottom graph), whereas if a yellow filter is used, the effect is repre- 

 sented by curve B. The area under curve A, compared with the area under B gives an 

 indication of the filter factor required. If the exposure with the filter is increased by the 

 filter factor, curve C results. 



In the spectral-distribution curves of Fig. 5, the top curve represents the spectral 

 distribution of the light source (mean noon sunlight, in this case), the second curve 

 represents the transmission characteristics of the filter (Wratten No. 15 or G filter), 

 and the third curve represents the sensitivity of a typical panchromatic film. The 

 bottom set of curves represents the net relative sensitivity of the panchromatic film 

 when used with the sunlight source (curve A), as well as the net effect of the photo- 

 graphic film when used with the same sunlight source but with the G filter which cuts 

 out all light below a wavelength of 500 m^ (curve B). The filter factor F is given by 

 the ratio of the area below the curve B to the area below the curve A, whereas the 

 exposure factor for the G filter (for mean noon sunlight and the panchromatic material 

 indicated) is given by the ratio of the area under curve A to the area under curve B. 

 If we increase the exposure by the proper filter factor, the results for the case in which 



