COLOR PHOTOGRAPHY 



617 



The additive process finds its application in the making of color transparencies. 

 It is also used sometimes for the projection of motion pictures in natural colors. 



The subtractive process can be used for transparencies and for motion pictures in 

 natural colors the same as the additive process. It is in addition the only successful 

 process which permits obtaining natural-color prints on paper to be viewed by reflected 

 light. From this particular standpoint the subtractive process is by far the most 

 important of all the photographic color processes as yet evolved. 



Ye/fow 



Green - - 



Blue green 

 cyan 



— Red 



Mogenia 



Blue 



Fig. 2. — Diagram showing mixing of pigments. 



Description and working instructions of the various subtractive and additive color 

 processes practiced today will be given in the order of their importance. 



Color-separation Negatives. — The production of color-separation negatives from 

 the original subject represents the first step in any color process. Three exposures are 

 necessary behind red, green, and blue filters, respectively. 



An exception to this rule is in the case of the so-called "color screen plates" or 

 films by the additive method in which the original negative, containing the three-color- 



100 



cS 2>Q 



m 



40 



20 



400 



450 



650 



500 550 600 



Wctveleng+h 



Fig. .3. — Transmission characteristics of Wratten tricolor filters 



700 



filter elements in mosaic form, is converted by chemical reversal to the final positive. 

 These color screen plates and films will be described later. 



The three primary-color filters necessary for making the three-color-separation 

 negatives must respond to certain definite requirements. Each filter must cover a 

 convenient section of the visible spectrum and must overlap into the region of the 

 adjacent filter by a moderate amount. The characteristics of these filters have been 

 extensively debated, but the recent investigations of D. A. Spencer^ have proved 

 conclusively that the present commercial standard filters, of which the Wratten 

 three-color filters are typical examples, represent the best possible compromise 



1 Spenceb, D. a., Phot. J., July, 1935, p. 377; Penrose Ann., 1938. 



