PHOTOGRAPHIC SENSITOMETRY 189 



precise methods in order to obtain a more simple specification expressible in a few 

 numerical terms of the spectral sensitivity of the photographic material. A common 

 numerical method of specifying the color sensitivity makes use of the relative sensitiv- 

 ity of the photographic material to a fairly wide range of wavelengths in a given region 

 of the visible spectrum. In order to isolate the desired regions of the spectrum, selec- 

 tive filters are commonly employed. 



Probably the most widely used and the most useful method of this type involves the 

 use of three filters having such selective absorption characteristics as to divide the 

 spectrum into three broad bands, each having approximately the same range of wave- 

 lengths. In the ideal case a set of tricolor filters for this type of work might have 

 complete transmission between the ranges of 400 to 500 myu, 500 to 600 m^u, and 600 to 

 700 ran, respectively, with very sharp cutoffs and complete absorption outside the 

 three transmission ranges. Such filters would not diminish the intensity of the light 

 in their transmission band because of the ideal transmission characteristics which 

 have been assumed. By exposing four test strips of a given photographic material 

 to a given light source properly, a tricolor specification for the spectral photicity of the 

 photographic material may be obtained. In order to do this, one of the test strips 

 should be exposed through each of the three tricolor filters, while the fourth strip 

 should be exposed directly to the light source without the use of any filter. The test 

 strips are processed in the usual manner, and the density of silver deposit on the four 

 strips is determined. The test strip for which no filter was provided will be the 

 densest, since all the spectral energy of the light source fell upon it. Let the density 

 of this test strip be Do. Let the density of the test strips be Di, Dz, and D3, for the 

 green, blue, and red filters whose transmission bands are 400 to 500 ran, 500 to 600 m^u, 

 and 600 to 700 m^, respectively. Then the tricolor sensitivity of the photographic 

 material (when used with the given light source) may be determined as follows: 



^ = 5i (38) 



^ = S2 (39) 



^ = Ss (40) 



In practice, of course, such ideal filters as have been assumed do not exist and do 

 not transmit unabsorbed all wavelengths in their transmission bands. Because of the 

 absorption in the transmission band, it is necessary to multiply the ideal factors or 

 sensitivities, as given above, by multiplying factors Ki, Ki, and i^s (each less than 

 unity), which factors represent the mean transmission of the practical filter in the 

 transmission region. 



A typical set of filters for tricolor work, such as might be used for determining the 

 tricolor sensitivity of photographic materials, are the Wratten No. 25, Wratten 

 No. 58, and Wratten No. 49 filters. The approximate factors for these filters are, 

 respectively, K\ = 0.8, Ki = 0.3, and K3 = 0.15. Consequently, if these filters are 

 used with the four test strips to give a specification of the tricolor sensitivity of photo- 

 graphic materials used with a given light source, the sensitivities may be expressed as 



^-S^ (41) 



^' = S, (42) 



^ = Ss (43) 



