PHOTOGRAPHIC MATERIALS 149 



used, provided its contrast characteristics were suitable for the subject and the 

 printing process to be used. As a matter of convenience, however, it may be desirable 

 to choose a material which has, as nearly as possible, equal filter factors for the three 

 tricolor filters. For most lighting conditions, this will mean a material with a type B 

 or orthopanchromatic sensitizing. 



Since the making of three successive exposures is ordinarily practical only for 

 still-life subjects and since satisfactorily accurate one-shot cameras are very expensive, 

 various bipack and tripack films have been developed, to allow one exposure in an 

 ordinary camera. 



The tripack consists of three films placed in contact and exposed simultaneously, 

 to make the three separation negatives. The first two films are placed with their 

 emulsion surfaces in contact, so that the front film, which is blue sensitive, is exposed 

 through the base. This emulsion is yellow-dyed, or carries a yellow-filter layer on its 

 surface, to prevent any blue light from reaching the two rear films. The second film 

 is orthochromatic and so makes the green-sensation record. It is on a thin support 

 and is backed with a red-filter layer so that only red light reaches the rear panchro- 

 matic emulsion. 



The two front emulsions must be as thin and transparent as possible, in order to 

 avoid excessive absorption of light, which would increase the exposure required, and in 

 order to reduce scattering of light which makes the rear image unsharp. The ratios 

 between the speeds of the three emulsions must be correct to compensate for the 

 absorption of light in the first and second emulsions, so that the one exposure will give 

 three well-matched negatives. It is desirable, but not necessary, to have the develop- 

 ment rates adjusted so that the three negatives can be developed together. 



Because of the scattering of light in the first two emulsions, and the separation of 

 the rear emulsion by the thickness of the film base, the loss of definition is such that 

 tripack negatives usually do not give satisfactory enlargements and so are more suit- 

 able for use in the large sizes of studio and view cameras than in the smaller hand 

 cameras and are more satisfactory for portraiture than for general commercial 

 work. 



To reduce this loss in definition, while still avoiding many of the complications of 

 the double-mirror cameras, various bipacks have been devised. The bipack is 

 essentially a tripack with the rear film removed and exposed directly to a second 

 image supplied by some type of beam splitter, such as a semitransparent mirror. 

 The front emulsion of the bipack, which is exposed through the base, makes the blue 

 negative, while the back emulsion may make either the green or red negative, depend- 

 ing on the design of the pack. In the Technicolor and Eastman bipack systems, the 

 red negative is made on the rear film of the bipack, and the green negative is made on a 

 separate film.^ 



Positive Materials. — To obtain a color print from the three separation negatives, 

 three positive images, in dyes or pigments of the respective complementary or minus 

 colors, are superimposed in register. Thus a yellow positive, which absorbs blue, is 

 made from the blue-filter negative, a magenta, which absorbs green, from the green- 

 filter negative, and a blue-green (cyan), which absorbs red, from the red-filter negative 

 (see chapter on Color Photography). These can be made by any of several processes. 

 A number of these, such as the bichromated gelatin, or so-called carbon process, the 

 gum-bichromate process, and their various modifications (particularly the widely 

 used carbro process) do not directly involve the use of sensitive silver halide emulsions, 

 and so they need not be described here. However, there are several methods in 

 which silver images are used to form dye or pigment images of the proper color. 



1 Ball,, J. A., The Technicolor Process of Three Color Cinematography, ./. Soc. Motion Picture 

 Engrs., 25, 127 (1935). 



