CHAPTER XXII 



COLOR PHOTOGRAPHY 



By Olindo O. Ceccarini 



Introduction. — It might be safely said that color photography began with the 

 formulation of the wave theory of light by Wtinsch and Young and the theoretical 

 work of Helmholtz and Maxwell. 



White light can be decomposed into a very large number of colored radiations, and 

 the colors constituting the visible spectrum range from violet to deep red. 



The three colors particularly striking to the human eye are blue, green, and red. 

 Maxwell demonstrated that a mixture of blue, green, and red lights in a suitable 

 , proportion produced the physiological sen- 



sation of white light. 



Mixture of Colored Lights. — In mixing 



colored lights the luminosity of the mixture 



^^i~gy^/Q,_^;^^^^i^(:^^^L-— -V^/Zow is greater than the luminosity of each color 



alone. Taking, for instance, blue and green 



lights, it is possible to obtain all the possible 



o/ a ^^^^^TttttTTI^^^^^ Green shades from the pure blue to the pure green 



by suitable variations of the individual com- 

 ponents. The diagram of Fig. 1 represents 

 the colors to be obtained by mixing lights 

 "CvcM" ^^ ^^^ "primary colors," blue, green, and 



„ r • • red, taken two by two and all three to- 

 FiG. 1. — Diagram snowing effect of mixing . . ^. -^ i i , j 



lights of "primary colors." gether in proportions suitable to produce 



white light. 



Yellow is evidentl}^ produced by the addition of red and green primary lights, and, 

 since it does not contain blue, it is called its "complementary," or "minus blue." 



In a similar manner the addition of blue and green lights gives blue-green (also 

 known as "cyan"). Cyan is evidently "complementary" to red, or "minus red." 



Red and blue lights produce "magenta" which is "complementary" to green, or 

 "minus green." 



Pigments. — The behavior of a mixture of colored pigments is entirely different 

 from that of the mixture of colored lights. For instance, a yellow pigment spread 

 evenly on a sheet of white paper, if observed with a spectroscope, shows practically 

 complete absence of blue light. The conclusion is therefore that the yellow "absorbs " 

 or "subtracts" blue from the white light reflected by the paper. If now a magenta 

 pigment is added on the yellow, the resultant color will be a deep red, for both blue 

 and green colors have been subtracted from the light of the paper. Adding now a 

 cyan pigment to the two already existing the result will be "black" or the complete 

 absence of color. 



The diagram of mixture of pigments is given in Fig. 2. 



The photographic analysis of a color object is carried out with filters of primary 

 colors, while the synthesis can be performed by mixture of lights also of primary 

 colors giving rise to the so-called "additive process" or by mixture of pigments in 

 complementarj' colors by means of the so-called "subtractive process." 



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