154 HANDBOOK OF PHOTOGRAPHY 



deposit of silver. The test strips are then dried, after which the densities of the 

 silver deposits for each exposure step on the same strip are measured. The values of 

 density and the corresponding values of the logarithm of exposure are plotted for each 

 strip. Similar plots are made for other strips having different times of development, 

 so that a family of cur\^es D = logio E is obtained. This family of curves gives most of 

 the essential information relating to the characteristics of photosensitive materials. 



The determination of the sensitometric characteristics of photographic materials 

 requires a knowledge and control of (but preferably standardization of) the following 

 conditions and apparatus: 



1. A light source emitting radiations of known spectral distribution and known 

 intensity. 



2. A means for producing a series of graded exposures of known magnitudes. 



3. Standardized conditions of processing (especially with regard to development) 

 the material under test. 



4. A method of determining quantitatively the density, graininess, and other 

 characteristics of the material after exposure and processing. 



5. A method of properly interpreting quantitatively the results of sensitometric 

 measurements. 



From standardization of 1 and 2 it is possible to determine the magnitude and 

 character of the stimulus (exposure) applied to the photographic material. Step 4 

 enables us to determine the characteristic responses of the photographic material to 

 the various stimuli provided by steps 1 and 2. Step 5 not only enables us to correlate 

 properly the stimuli and its response, but enables us to express the causal relations in 

 such manner as to permit of optimum use of photographic materials, filters, light 

 sources, and other accessories. An understanding of photographic sensitometry is 

 also useful in recognizing and correcting faults of over- or underexposure, incorrect 

 rendition of tone, incorrect rendition of colored objects in monochromatic or color 

 photography, etc. 



Application of Sensitometry. — Originally photographic sensitometry was used 

 almost exclusively by manufacturers of film material for the determination of film 

 characteristics and by a few scientific workers using photographj^ The first impetus 

 toward extensive general uses of photographic sensitometry came from the motion- 

 picture field with the introduction of sound pictures. While sensitometric methods 

 and processes are not yet employed by the average photographer or serious amateur 

 (nor are they likely to be used except in the simplest fundamental steps), a knowledge 

 and understanding of sensitometric methods may not onlj^ be advisable in aiding the 

 proper exposure and processing for portraits, pictorial, and other subjects but may be 

 essential — or verj^ nearly so — for motion-picture work, certain kinds of scientific 

 photography, and color, aerial, and other branches of photography where exacting 

 requirements must be met. 



Light Sources for Photographic Sensitometry. — The first step in photographic 

 sensitometry involves giving the photosensitive material a series of standard (or at 

 least precisely known) exposures. The exposure of the material under accurately 

 known conditions is accomplished by means of a sensitometer which consists, func- 

 tionally, of (1) a standard source of light or other radiation and (2) some form of 

 exposing device, such as an accurate shutter, which determines the quantity of light 

 which is permitted to act on the photosensitive material. 



We shall first give some consideration to the light sources and the exposure devices 

 suitable for sensitometry. In a later section consideration will be given to sensitom- 

 eters in which the light source and exposure device are combined. 



Practical Light Sources.— T)\e most important of a wide variety of light sources 

 which have been used for photographic sensitometry include, in approximately 



