358 HANDBOOK OF PHOTOGRAPHY 



Such methods may be divided into classes: 



1. Inspection methods, with or without desensitization. 

 a. Image nearly developed. 



6. Initial image appearance (Watkins factor). 



2. Time-temperature methods (thermal system). 



3. Sensitometrie and test exposure control. 



Inspection Methods. — Judging development by visual inspection of the image as it 

 progresses and grows is one of the oldest procedures of the art of photographJ^ It 

 was particularly necessary with the handicaps of emulsion variations and nonuniform- 

 ity under which the photographers of an earlier day labored. It is still an important 

 method for practical work, though the burden of processing control is shared by other 

 methods in many laboratories, particularly those using photography for scientific 

 purposes and those handling large amounts of bulk work. Progress in manufacturing 

 control of emulsion uniformity has been an extremely important factor in influencing 

 this trend. 



Two separate lines of attack have been pursued in the use of inspection 

 methods for development control. In one system, known as the Watkins factorial 

 method, development is based upon the experimental observation that for one emul- 

 sion and developer there is a constant ratio between the time to reach a given contrast 

 and the time of appearance of the image. This may be restated in the form 



Time of development = time of appearance X constant 



where the time of development refers to the development time for some contrast 

 selected as suitable for the work and the time of appearance is the time between immer- 

 sion of the emulsion in the developer and first appearance of the image. The constant 

 is known as the Watkins factor and varies from emulsion to emulsion and from devel- 

 oper to developer. To operate this system adequately, the time of appearance of the 

 developed image must be carefully determined. This is relatively easy with slow, 

 noncolor-sensitized emulsions but becomes more difficult as the emulsion speed is 

 increased. This fact tends to direct the use of the method toward the slower emul- 

 sions, such as the paper, lantern-slide, and slower negative emulsions. 



The second and older line of inspection control is based upon judgment of the 

 image as it approaches the density desired. It is subject to many of the limitations 

 encountered in the Watkins system but has many exponents. Experience is required 

 when changing from emulsion to emulsion, as the darkroom appearance of images 

 depends upon the emulsion used and not upon its contrast and density alone. It is 

 most readily applied to the slower emulsions where illumination levels may be higher 

 than with high-speed emulsions. It is most valuable in the hands of a thoroughly 

 experienced worker, where compensation in development is tolerable, and may lead to 

 better final results than absolute uniformity of treatment. 



Desensitization Methods. — Both of the methods enumerated above maj^ be modified 

 by the use of desensitizers with color-sensitized emulsions to permit more illumination 

 in the darkroom and hence better inspection facilities. Many compounds have been 

 found which have desensitizing action, but not all are suitable. Some produce fog 

 and others leave the latent image in such condition as to be particularly subject to 

 bleaching by darkroom lights used. Phenosafranine, pinakryptol green, and pina- 

 kryptol yellow are the most common desensitizers. They are not directly inter- 

 changeable, but any of them may be used in a separate bath prior to .development. 

 Some difficulty has been experienced in producing a stable developer with pheno- 

 safranine and hydroquinone in the same solution. No such trouble has been found 

 with pinakryptol green, and this compound appears as the one of the three most 

 available for use directly in the developer, as its presence was not foimd to influence 



