PHOTOGRAPHIC MATERIALS 125 



For this reason it greatly extends the straight-line portion of the characteristic curve 

 for the combination. 



The slow bottom emulsion also reduces the effect of halation to a considerable 

 extent, because it absorbs part of the light that would otherwise get through to the 

 base and also, being less sensitive, it is affected much less by the light which is reflected 

 from the rear surface of the base. 



Overcoating. — The grains of the emulsion can be made developable by mechanical 

 action as well as by light action, so that scratches and abrasions show up as dark 

 streaks after development. In order to prevent the formation of such defects during 

 handling, films are often provided with a thin top coating or overcoating of clear 

 gelatin. Film-pack emulsions are practically always overcoated, as are some roll 

 films. Since the overcoating hinders diffusion of the processing solutions to some 

 extent, an overcoated emulsion usually develops more slowly than a similar emulsion 

 without the overcoat. This is why certain film packs require a longer time of develop- 

 ment than the corresponding emulsions on roll film. 



Paper emulsions are often particularly sensitive to abrasion, so that they are often 

 given an overcoat or "antiabrasion" layer. 



Strip-ping Film. — In certain special processes, particularly in the graphic arts, it is 

 desirable to remove the emulsion from its original support and transfer it to a new 

 support, either so that it can be trimmed as desired and combined with portions from 

 other negatives or so that it can be reversed, right for left. Emulsions for this purpose 

 are generally coated on a thin skin of hardened gelatin or cellulose nitrate or acetate, 

 to give them sufficient mechanical strength for handling after stripping. This skin is 

 cemented to the support by an adhesive which holds it in place until stripping is 

 desired. 



Characteristics of Photographic Materials. — The methods of measurement and 

 the significance of the various relations between exposure, development, and density — 

 i.e., the sensitometric characteristics, such as emulsion speed, contrast, latitude, fog, 

 etc. — are discussed in the chapter on Photographic Sensitometry, and some values are 

 given below in the discussion of specific materials. 



In addition to the speed, contrast, rate of development, etc., there are several 

 other properties to be considered in the selection of materials for any particular 

 purpose. Of these, the spectral sensitivity and the graininess and resolving power are 

 probably the most important. 



Spectral Sensitivity. — The normal human eye is sensitive to radiant energy over 

 the wavelength range from approximately 400 to 760 m/x, or from violet to red, with 

 the maximum of sensitivity to the yellow-green, at around 550 (Fig. 5). 



The spectral sensitivity of the photographic emulsion, however, is quite different, 

 beginning far in the ultraviolet and overlapping the visual sensitivity only in the blue. 

 The different silver halides vary slightly, but all are practically completely insensitive 

 to green, yellow, and red. Therefore ordinary emulsions are said to be color blind, 

 because they reproduce colored objects in very different brightness relations than are 

 seen by the eye. 



This normal sensitivity of the silver halide can, however, be greatly extended by 

 the use of certain sensitizing dyes. At first, commercial emulsions were sensitized 

 only into the green. This gave a very noticeable improvement in the rendition of 

 colored objects but hardly justified the name "orthochromatic" (true color) which was 

 applied to distinguish these improved emulsions from the ordinary color-blind mate- 

 rials. Later, when really "true color" emulsions were produced, which were sensitive 

 to the red as well as the blue and green, they were called "panchromatic" (all color). 

 The first panchromatic emulsions had only low sensitivity to the green and red as com- 

 pared to the normal blue sensitivity, but, as new and better sensitizing dyes became 



