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HANDBOOK OF PHOTOGRAPHY 



The filters usually employed are of the Wratten type put out by the Eastman 

 Kodak Co. Since more than 100 different forms are available, a comprehensive treat- 

 ment is impracticable here, but a few examples will be cited, together with sufficient 

 theory to make these instances intelligible.^ 



In Fig. 40, top, the reader will note a wedge spectrogram of Eastman pan- 

 chromatic negative film as already described under Films and Film Characteristics, 

 taking in the full band from about 400 to about 670 m/x. The effect of a typical 

 filter, the light yellow Wratten No. 3 (Aero 1), used with daylight illumination, is 

 shown in the bottom spectrograni. The longer Avavelengths are unaffected, but the 



Wedge spectiogiam of 

 film without filter 

 (daylight) 



0.1 



Spectrophotometric 

 absorption curve of No. 3 

 filter 



S 10 1 



100 



200 



300 



400 500 



Wave Length 



600 



700 



Wedge spectrogram of 

 film with filter (daylight) 



Fig. 40. — Effect of Wratten No. 3 filter on Eastman panchromatic film with daylight. 



shorter wavelengths are cut oft' in the neighborhood of 450 m^u. The spectrophoto- 

 metric absorption diagram in the center shows how the filter produces this effect. The 

 white area of such a diagram is read downward logarithmically in transmission. per- 

 centage, while, conversely, absorption is read upward logarithmically in the black 

 area. Black represents the absorption of the filter, while white shows its transmission. 

 Ignoring the slight transmission in the ultraviolet, we note that this filter cuts out the 

 ultraviolet, violet, and a portion of the blue up to 450 \nix, and passes the rest of the 

 visible spectrum. Hence this filter would be used whenever suppression of the short- 

 wave side of the total band is desired. 



Since the function of the filter is, by absorption, to prevent a portion of the light 

 from reaching the film, it necessitates an increase in exposure corresponding to the 

 degree of absorption. The number of times the exposure must be increased with a 



1 For full data on specific filters see "Wratten Light Filters," published by the Eastman Kodak Co. 



