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HANDBOOK OF PHOTOGRAPHY 



which tend to cause the material to curl. The early films, coated on a comparatively 

 thin base, often curled into a ^Tight roll no larger than a pencil. In order to prevent 

 this curling, most films are now coated on the back with a layer of hardened gelatin, 

 which has practically the same shrinkage as the emulsion and so balances the strains. 

 Antihalation Backings. — When a beam of light enters the emulsion, it is scattered 

 to some extent by successive reflections from the surfaces of the silver bromide grains, 

 causing irradiation, as has been described. At the brighter points in the image the 

 emulsion cannot absorb all the light, and a considerable portion passes into the base. 

 Of this light, the rays which strike the back of the support nearly normal to the surface 

 pass out into the air and do no harm, provided the plateholder or pressure plate has a 

 nonreflecting surface. However, all rays striking the rear surface at less than a cer- 

 tain critical angle undergo total internal reflection and pass back through the base, 



Fig. 4. — Diagram illustrating how halation is produced in ordinary emulsions by surface 



reflections. 



reaching the einulsion at some distance from the original image. Thus, around every 

 bright point of the image, there is produced a circle or halo of light, with a diameter 

 depending on the thickness of the base (hence it is smaller for film than for glass). 

 This effect, called "halation," is illustrated in Fig. 4. The halation can be prevented 

 by placing some material between the emulsion and the rear air surface which will 

 absorb the troublesome light. Thus the base can be dyed a light gray, and, since the 

 halation-producing light passes twice through the emulsion at a considerable angle, 

 it is greatly reduced in intensity, while the printing light, which must pass only once, 

 is only slightly affected. With films which have a gelatin backing, however, it is 

 more customary to color the backing with a dye, or a pigment, which is decolorized 

 during treatment in one of the usual photographic solutions, such as developer or 

 fixing bath. Since it is only necessary to absorb light to which the emulsion is sensi- 

 tive, orthochromatic materials are usually provided with a red backing which absorbs 

 blue and green, while panchromatic materials are provided with a dark-green backing 

 which absorbs chiefly red and blue but also some green. 



Double Coating. — In order to increase the exposure latitude or the brightness 

 range which they can record, some films (and plates) are coated with two thicknesses 

 of emulsion. The bottom emulsion is usually slower than the top emulsion, so that it 

 is not affected until the exposure approaches the shoulder region for the top emulsion. 



