200 HANDBOOK OF PHOTOGRAPHY 



consequently viewed by transmitted light. Such materials require little comment at 

 this time, since their characteristics resemble those negative materials which have 

 already been discussed. They differ from negative materials mainly in that their 

 speeds are lower, and the maximum values of their gammas are usually higher than 

 those of negatives. Spectral sensitivity, latitude, grain, etc., of positive materials 

 may also differ from those of negative materials. The other classification of positive 

 materials includes opaque materials, such as printing papers, which must be viewed 

 by reflected light. Their characteristics are sufficiently different from those of nega- 

 tive materials to warrant some consideration at this point. 



Printing papers of the bromide type, designed for making enlarged prints from a 

 negative, have emulsions which resemble those of negatives; consequently the D-logio E 

 characteristics are similar to those of negative materials. The density range and 

 sensitivity are somewhat more restricted than in the case of negatives, but, like nega- 

 tive materials, the gamma increases with the development time, although only to a 

 small extent. 



The characteristics of chloride papers, which are most frequently employed in 

 contact printing, differ considerably from those of bromide papers. The gamma of 

 chloride papers is much less affected by the development time than the gamma of 

 bromide papers, and development extended beyond the time required to produce 

 gamma infinity acts so as to change the effective speed rather than the gamma of the 

 chloride material. 



The chief differences between the characteristics of negatives and the correspond- 

 ing characteristics of printing papers are: 



1. The printing paper is opaque, and consequently must be viewed by reflected 

 light. 



2. As a consequence of the above, the blackness of the silver deposit is measured 

 by the reflection of light from the surface of the paper, rather than by passing light 

 through this material, and this density is spoken of as the reflection density. 



3. The density range of printing papers depends to a large degree upon the surface 

 texture but is usually less than the density range of negative materials and is often 

 between 1.5 and 2.0. 



4. The D-logio E characteristics of the printing-paper materials have a shorter 

 straight-line region than that of many negative materials. In some cases no linear 

 region may exist. 



5. The gamma infinity of printing papers, especially chloride papers, is deter- 

 mined largely by the characteristics of the emulsion coating of the paper and, to a 

 relatively small extent, by the conditions of development. 



6. As a result of (5), variations in contrast of printing are secured by the selection 

 of a suitable grade of printing paper rather than through any conditions of processing 

 of printing papers. 



The density of printing papers is defined in a manner analogous to that used for 

 negatives. However, since we must deal with refiected rather than transmitted light, 

 certain minor modifications are required in specifying the density of printing papers. 

 If /o is the luminous flux reflected from a perfectly white surface (such as magnesium 

 carbonate, which is frequently used as a reference) and if Ir is the light flux reflexjted 

 from the paper having a black deposit of silver, then the amount of light reflected by 

 the silver deposit or the reflecting power of the developed paper is 



R=j- (51) 



The density of the developed silver deposit is then defined as 



D = log 10 (y) ^ ^^S" \e) ^ "l^Sio R 



