PHOTOGRAPHIC SENSITOMETRY 



191 



Several methods for assigning to every kind of emulsion an arbitrary number to 

 serve as an index of its relative speed have been devised. In each system the number 

 increases with the speed or sensitivity of the emulsion, but the different systems are 

 rather inconsistent and do not produce results which are comparable with one another 

 because of the different speed criteria used. The most widely used systems for deter- 

 mining sensitivity of emulsions are based on: 



1. The threshold speed, which was the earliest method of specifying film or plate 

 characteristics. 



2. Inertia speed, a later method introduced by Hurter and Driffield. 



3. Latitude-inertia speed, which is coming more and more into common use. 



4. Minimum useful gradient speed. 



2.0 



-2.0 



-1.0 







Log 10 ^ 



1.0 



2.0 



Fig. 24. — Z)-logio E characteristic and the exposure values from which a number of well- 

 known speed systems are determined. This curve is used only for purposes of illustration 

 and does not represent the characteristic of any particular material. 



Threshold-speed Ratings. — In methods of sensitometry which do not involve the 

 measurement of the densities of the developed strip, the only possible means of deter- 

 mining speed is to note the exposure required to produce a density which is just 

 perceptible. Actually fog density is always present to a greater or lesser extent and 

 will contribute to the minimum perceptible density, so that what is usually determined 

 is the minimum perceptible density difference due to exposure and fog. A further 

 disadvantage of this method is that the minimum perceptible density depends greatly 

 upon the conditions under which inspection is made. Consequently, unless the 

 density determinations are made under carefully controlled and standardized condi- 

 tions, the values of threshold speed obtained by different observers, from the same test 

 strip, may differ widely. 



The earliest method of determining the sensitivity or speed of photographic mate- 

 rials made use of the threshold sensitivity. By threshold sensitivity is meant the 

 minimum exposure which will result in a detectable deposit of silver, as represented by 

 the exposure Es in Fig. 24. This method is based on the assumption that an emulsion 

 having a low threshold exposure will be a relatively fast emulsion. The most famihar 

 speed ratings based upon the threshold speed are the Scheiner ratings, the Eder-Hecht 

 speeds, and the DIN numbers, of which the latter is a modification of the threshold- 

 speed method. 



