140 HANDBOOK OF PHOTOGRAPHY 



Paper Prints. — The great majority of photographic prints are made on paper and 

 are viewed by reflected Ught. This has two important effects. First, a certain frac- 

 tion of the incident light is reflected by the emulsion surface and the grains near the 

 surface; this reflected light definitely limits the maximum black that can be obtained. 

 Thus, with some matte-surface papers where the diffuse reflection is considerable, the 

 maximum black reflects as much as 6 per cent of the incident light, thus giving a total 

 brightness range from high lights (represented by the clear paper base) to shadows of 

 only 15 to 1 or a density range of 1.18. With certain glossy papers, on the other 

 hand, where the diffuse reflection is low, the total density range may be as much as 

 1.7 or more. This effect definitely limits the tone range that can be reproduced by 

 any particular paper. Fortunately, the brightness range, at least of the important 

 parts, of normally lighted subjects is usually not greater than about 1 to 40 and so can 

 be reproduced satisfactorily on paper. While the maximum density varies also with 

 the particular type of emulsion, average values for the various surface finishes are as 

 follows : 



Glossy 1.5-1.7 



Semimatte About 1 . 4 



Matte About 1 . 2 



The manner in which the light is reflected depends upon the state of the surface. 

 With an optically smooth or "glossy" surface, the angle of reflection is equal to the 

 angle of incidence, as with a mirror, and little or no light is scattered in other directions. 

 With an optically rough surface, on the other hand, the reflected light is scattered in 

 all directions. In the case of a photographic print, or any other picture on paper, it 

 is the "diffuse" or scattered reflection which limits the maximum density, since the 

 print can be turned so that the specular, or mirrorlike, reflection does not reach the 

 eye. In measuring the diffuse reflection, in order to calculate the density, the paper is 

 usually illuminated at an angle of 45° and viewed normal to the surface. 



The second effect is that, in the high lights and middle tones, the light passes 

 through the emulsion to the base and then is reflected back through the emulsion. 

 Therefore each area of the silver deposit has twice the density it would have for trans- 

 mitted light. Thus, since the maximum density is already limited by reflection, paper 

 emulsions are coated very much thinner than emulsions of film or glass. 



For papers, three types of emulsions are used, differing in the composition and 

 grain size of the silver halide; bromide, for black tones; chloride, for black tones; and 

 chlorobromide of very fine grain for warm tones on direct development. 



Speed. — Paper emulsions cover a wide range of speeds, the ratio between the 

 slowest and fastest paper emulsions being considerably greater than the ratio between 

 fast papers and the fastest negative materials, as is shown by the approximate values 

 in the table. 



Table VI. — Relative Sensitivity of Various Photographic Materials 



Type of Emulsion Relative Sensitivity 



Positive emulsions (lantern slide, etc.) 1 , 000-3 , 000 



Bromide papers 300-1 , 000 



Chlorobromide papers, warm tones 100-200 



Special warm-tone lantern slides, according to tone desired 1-25 



Chloride papers 1-5 



Bromide emulsions are comparatively fast, having speeds only slightly less than 

 the positive films, and so are suitable for printing by projection. Chloride papers are 

 very slow and so are normally suitable only for contact printing. The chlorobromide 

 papers cover a considerable range in speed, depending on the composition. The slower 

 ones, with speeds similar to the chloride papers, are suitable only for contact printing. 



