166 HANDBOOK OF PHOTOGRAPHY 



Development for Sensitometry. — The sensitometric strips are developed after 

 exposure. Since the density of the silver deposit depends upon the developing condi- 

 tions as well as upon the exposure, it is apparent that the developer and the conditions 

 of development must be standardized for precise sensitometric work. The conditions 

 which are chosen for standardization, however, cannot be established once and for 

 all, for consideration must be given to the type of work being accompHshed. The 

 development conditions will obviously be different for research work on developers 

 than for routine processing of a certain class of film. 



The investigations of various workers have shown that the constitution of the 

 developing solution affects such photographic characteristics of a given material as 

 rate of development, contrast, density, fog, latitvide, speed, etc. A paper by Clark^ 

 summarizes the effect of changes in the constitution of developers and may be con- 

 sulted by those interested. 



Developers for Sensitometry. — For a comparison of emulsion characteristics and for 

 investigations in which the control of processing is not involved, standard developers 

 and developing conditions may be established for photographic sensitometric work. 

 A developer suitable for this class of work should have the following characteristics: 



1. Good reproducibility 



2. Low temperature coefficient 



3. Low sensitivity to changes in concentration of bromide 



4. Production of neutral density image free from stains and selective absorption. 



5. Good keeping qualities. 



For many years a pyro developer was used for sensitometric work. Such a devel- 

 oper was generall}'^ suitable since pyro developers were extensively used in practice. 

 However, since some of the reaction products of pyro developers are definitely colored 

 and are absorbed by the film, the developed negative has a yellowish stain which has 

 considerable absorption to the blue, violet, and ultraviolet. The photographic and 

 visual density are therefore likely to be quite different and corrections are required 

 when converting one density into the other. Pyro developers have fallen somewhat 

 into disuse during recent years, so that a pyro developer for sensitometric work does 

 not have the merit it formerly possessed. 



A developer suggested by Sheppard and Travelli,^ suitable as a standard developer 

 for scientific sensitometric work, consists of: 



p-Aminophenol hydrochloride 7 J-l g. 



Sodium sulphite (anhydrous) 50 g. 



Sodium carbonate (anhydrous) 50 g. 



Water to make 1000 co. 



This developer produces a neutral-gray deposit, has desirable fog characteristics, 

 contains no bromide, and is relatively insensitive to the presence of bromide so that the 

 regression of inertia with increase in time of development and speed should be inde- 

 pendent of development time. This developer has a relatively high temperature 

 coefficient,^ so that for precise sensitometric work the temperature must be controlled 

 accurately during development. 



1 Clark, W., Phot. J., 65 (n.s. 49), 76 (1925). 



2 Jones, L. A., "Photographic Sensitometry," Eastman Kodak Co. (19.34). 



' The temperature coefficient for a particular developing agent is defined as the ratio of the velocity 

 constants k for any two temperatures. The difTerence in temperatures for the two velocity constants 

 is chosen as 10°C. Thus, if A20 is the velocity constant at 20°C., and fcao is the velocity constant at 

 30°C., the temperature coefficient t of this developer between 20° and 30°C. will be 



r = ^" (10) 



