162 HANDBOOK OF PHOTOGRAPHY 



experimentally observed facts, especially for low-intensity exposures, expresses the 

 exposure 1 E in terms of the luminous intensity of specified distribution / and the total 

 time of exposure t in the form 



E = /"'r (8) 



where the exponents may have A^alues greater than unity. Some investigators have 

 found more complicated relations in which it was necessarj^ to express the exposure 

 in terms of the characteristics of the photographic material under investigation. 

 Obviously this is an undesirable situation, and it is convenient to have the various 

 factors involved as independent of one another as possible. 



Since the failure of the law of reciprocity and the intermittency law do not produce 

 appreciable errors for the ordinary range of luminous intensities and exposure times 

 used in sensitometry (or in general photography, for that matter), the exposure has 

 been defined to be 



E = It (9) 



This is a simple relation for the exposure and is based on the assumption that the 

 exposure is of continuous duration. This definition of exposure, which is used in 

 sensitometry, has the additional advantage that effects of intermittency can be 

 studied independently, using the continuous exposure, as employed in ordinary sensi- 

 tometric methods, as a standard of comparison. In photographic sensitometry the 

 exposure is expressed in meter-candle-seconds unless otherwise stated. 



Methods of Making Exposure. — The adoption of the above definition of exposure 

 makes it evident that the graduated exposure of sensitometric strips may be controlled 

 in any one of three ways: 



1. By keeping the intensity / constant and varying t. 



2. By keeping t constant and varying the intensity /. 



3. By varying both / and t. 



In these three cases it is assumed that the spectral-energy distribution of the 

 luminous source remains unchanged. 



For simplicity in the construction and use of sensitometers, onh"- the first two meth- 

 ods are in common use, and the first two classifications enumerated above give rise to 

 the two classifications of sensitometers or exposure devices in common use. If the 

 illumination is maintained constant and exposure is varied by altering the time 

 throughout which exposure takes place, a time scale of exposure is obtained. A 

 sensitometer operating on this exposure principle is termed a "time-scale sensitome- 

 ter." On the other hand, if the time during which the exposure is made is kept con- 

 stant and the exposure is A'^aried by altering the intensity of illumination between 

 successive steps, an intensity scale of exposure results. A sensitometer \ising this 

 principle is commonly referred to as an intensity-scale instrument. Both the inten- 

 sity-scale and the time-scale sensitometers vaa,y be made to produce a sensitometric 

 strip which is either continuous or stepped in its density variation. 



Following Jones, ^ we may classify exposure devices in the following manner: 

 Type I: Intensity-scale instruments 

 I variable, t constant 



1. Continuously varjdng exposure 



2. Stepped exposure 



1 In this case, we use the term exposure (for want of a better name) to indicate the intensity-time 

 product function, which will be integrated by the photographic material so as to give the same effect 

 as if the law of reciprocity was exactly followed and the intermittency law was nonexistent. 



2 Jones, L. A., Photographic Sensitometry, /. Soc. Motion Picture Engrs., 18, 32 (1932). 



