PHOTOGRAPHIC SENSITOMETRY 161 



Solution A 



Copper sulphate 3 . 707 g. 



Mannite 3 . 707 g. 



Pyradine 30.0 cc. 



Distilled water to make 1000 . cc. 



SOLTTTION B 



Cobalt ammoniuin sulphate 26 . 827 g. 



Copper sulphate 27 . 180 g. 



Sulphuric acid (sp. gr. 1 .835) 10.00 cc. 



Distilled water to make 1000 . cc. 



Exposure Devices. — In making photographic sensitometric measurements, it is 

 customary and convenient to subject several strips of the material to be tested to a 

 series of graded exposures so that the developed test strip contains varying amounts 

 of silver deposit from the minimum (represented by fog density) to the maximum 

 possible for the type of material under test. Usually the test strips are exposed in a 

 series of discrete or stepped exposures varying in geometrical ratio, and the developed 

 test strip is sometimes spoken of as a scale of grays. To expose a test strip of this 

 type some type of exposure modulator or graduated exposure-determining device is 

 required. Furthermore it is essential to know the manner in which the density of the 

 silver deposit builds up with the duration of the exposure time, as well as with the 

 manner in which the photographic material is exposed. 



Reciprocity Law and Intermittency Effects. — It is usually assumed that the photo- 

 chemical reaction of photographic materials is directly proportional to the total 

 energy affecting the material and that the exposure E is equal to the intensity of the 

 radiations / impinging upon the plate and the time t during which these radiations 

 act. As a result of this assumption, it is implicitly assumed that the photochemical 

 effect is independent of the absolute value of either factor, I or t and, consequently, 

 that the photographic materials properly integrate the effects of incident radiant 

 energy. To a first approximation, this assumption is true. This is the reciprocity 

 law.^ 



The assumption is not exactly true for photographic materials, however, and while 

 the failure of the reciprocity law is usually insignificant for most applications of 

 photography, it cannot be neglected where extremely long or extremely short exposures 

 are given. The intermittency effect, by which is meant the failure of a photographic 

 material to record the same density for a continuous exposure as for intermittent 

 exposure of the same total energy, is closely related to the failure of the reciprocity 

 law. Like the failure of the law of reciprocity, the intermittency law does not produce 

 appreciable errors for the majority of applications to which photosensitive materials 

 are used. However, neither effect can be neglected where precise results are desired, 

 such as in photographic sensitometry, or where very short or very long exposures are 

 involved. 



Both of the effects mentioned above must be given consideration in photographic 

 sensitometry since these factors affect the design and construction of suitable sen- 

 sitometric exposure devices. An exposure device operating continuously does not 

 produce the same effect as an intermittent exposure device which permits the same 

 total energy to be impressed on the photographic materials. These two factors also 

 affect the definition of exposure for sensitometric work, and the proper interpretation 

 of sensitometric data. 



Several formulas have been derived to express the response of the photosensitive 

 material for those cases where intermittency effect and failure of the law of reciprocity 

 need be considered. Of these, one of the simplest which agrees reasonably well with 



1 J. H. Webb has published a number of important papers on the reciprocity law in the Journal of 

 the Optical Society of America. 



