CHAPTER VII 



PHOTOGRAPHIC SENSITOMETRY 



By Bevehly Dtidley 



Introduction. Meaning of Photographic Sensitometry. — -The term "photographic 

 sensitometry " is frequently used in a restricted sense to specify the sensitivity of 

 photographic materials to visible light or other radiations. A somewhat broader 

 view will be taken here. We shall use the term photographic sensitometry to denote 

 the quantitative determination of all the various characteristic responses of the photo- 

 graphic materials to radiant energy. By radiant energy we shall usually mean 

 visible light. 



According to this definition, photographic sensitometry deals with those character- 

 istic responses which are produced by: (1) the intensity and spectral distribution of 

 the source of radiant energy acting upon the photosensitive materials, (2) the total 

 duration of time and the manner in which the photographic materials are exposed to 

 radiant energy, and (3) the processing (developing, fixing, washing, drying, etc.) to 

 which the photographic materials are subjected. 



The characteristics of photographic materials which may be determined through 

 sensitometry include: (1) the response to the intensity of the source of energy to which 

 the material is exposed, usually specified in some "speed" or "sensitivity" rating; (2) 

 the spectral sensitivity, or the relative response to energy at various wavelengths; 

 (3) the response to exposure of continuous duration; (4) the effect of intermittent 

 exposure, involving the manner in which photographic materials integrate the energy 

 to which they are exposed; (5) the response of the photographic material to varying 

 conditions of processing (especially development) for specified exposure; and (6) the 

 graininess. According to our definition such matters as the resolving power of the 

 photographic material and the distortions of the images which occur also properly 

 belong to the province of photographic sensitometry. However, these matters are 

 not of sufficient general importance and interest to be treated in this chapter, although 

 they may be of considerable practical importance in certain branches of photography, 

 such as astronomical photography or where images must be reproduced accurately 

 to scale. 



Photographic sensitometry may be used to determine the characteristics of photo- 

 graphic materials which reproduce colored images as well as those materials which, 

 for want of a better name, may be termed monochrome, and which produce images in 

 varying densities without any appreciable spectral selectivity. The sensitometry of 

 materials for color photography is somewhat more involved, elaborate, and extensive 

 than that required for monochrome photography, but the fundamental principles 

 involved are identical in both cases. 



In general, the procedure for the determination of sensitometric data is as follows : 

 Test strips of the material under examination are given precisely known, graded 

 exposures to a light source of specified intrinsic luminous intensity and spectral dis- 

 tribution. These strips are developed for specified lengths of time under known 

 conditions of temperature, in a developer of known composition and concentration. 

 The developer used depends upon the purpose for which the sensitometric measure- 

 ments are made, but it is usually one which produces an unstained neutral-gray 



153 



