148 HANDBOOK OF PHOTOGRAPHY 



When sufficient exposures can be given, duplicates can be produced directly by 

 making use of the solarization region of the characteristic curve. Duplicating films 

 and papers are available which have the whole emulsion made developable by treat- 

 ment during manufacture. Exposure to light reduces the developable density, so 

 that exposure behind a negative gives a negative, or behind a positive a positive.^ 



X-ray Films. — Because of the very low absorption of X-rays by the silver halide 

 of the emulsion, only a small portion of the incident radiation is effective in producing 

 a developable density. In order to increase the absorption and thus the photographic 

 eifect, emulsions for use with X rays contain a high percentage of silver bromide and 

 formerly were very thickly coated. Because of the difficulties encountered in devel- 

 oping and fLxing these thick emulsions, X-ray ffims are now given emulsion coatings of 

 normal thickness on both sides. 



Even with emulsions very rich in silver bromide, when the more penetrating 

 X rays are used, the direct action on the photographic emulsion is comparatively 

 slight, and it is now customary to place the ffim between intensifying screens coated 

 with a substance such as calcium tungstate, which fluoresces under the action of 

 X rays. Thus the greater part of the exposure is due to visible light from the intensi- 

 fying screens. 



Since the radiographs are often needed in a hurry, particularly in emergency 

 surgical cases, and since it is often not possible to keep the processing solutions down 

 to the recommended temperature. X-ray emulsions are usually hardened to a greater 

 degree than most negative films, in order to reduce the risk of trouble or damage from 

 excessive swelling. 



Emulsions for Color Photography. — Because of the great differences between the 

 various systems of color photography, the characteristics required of the emulsions 

 used also vary greatly. 



Additive Processes. — The only additive processes of practical importance at the 

 present are the integral screen-plate processes such as the Autochrome plates and the 

 Agfacolor and Dufaj^color films, which are processed by reversal, and the Finlay 

 process which uses separate taking and viewing screens. The requirements of the 

 reversal processes have been discussed. The Finlay process, which uses compara- 

 tively coarse mosaic screens, employs a regular fast panchromatic plate for the nega- 

 tive, the only requirement being that the color sensitivity be sufficiently uniform to 

 remain properly matched to the taking screen and compensating filter. The plate 

 exposed behind the taking screen in an ordinary camera is developed to a negative in 

 the regular manner. Prints are made on positive plates, similar to lantern-slide 

 plates, which are then bound in register with the viewing screen which has the same 

 pattern as the taking screen. Thus as many positives as desired can be made from 

 one negative, also there is a chance to control the contrast and to compensate for 

 errors in exposure of the negative by controlling the exposure and development of the 

 positive. This and other advantages claimed over the reversal processes are some- 

 what offset by the larger size of the individual screen elements. 



Suhtractive Processes. — The materials used in the various subtractive processes 

 can be divided into negative, positive, and reversal. Color-separation negatives 

 obtained by any of the taking systems can in most cases be used with any one of 

 several printing systems, while the reversal system gives the color photograph directly, 

 as with the additive processes. 



Negative Materials. — The simplest sj^stem, from the viewpoint of demands on the 

 emulsion, is the production of the three color-separation negatives by successive 

 exposures in an ordinary camera. Practicalljr any panchromatic emulsion could be 



■ Barth, W., a Film Emulsion for Making Direct Duplicates in a Single Step, J. Soc. Motion 

 Picture Engrs., 27, 419 (1936). 



