PHOTOGRAPHIC SENSITOMETRY 193 



DIN system, the reference value adopted is the exposure required to produce a density 

 of 0.1, i.e., an opacity of 1.26 or a transmission of 0.79, represented by the point Ed on 

 the curve of Fig. 24 when the test strip is subjected to a specific development pro- 

 cedure. This criterion obviates the first objection to the Scheiner and Eder-Hecht 

 systems, since it determines the index from a precise, definitely specified and measured 

 density rather than from an estimation of barely perceptible fogging. In practice the 

 emulsion is exposed through a step wedge, which is a plate in which successive steps 

 differ in density by a constant amount. The various steps are numbered, and the 

 number corresponding to the step which produces a density of 0.1 is the DIN number. 

 Like the Scheiner and Eder-Hecht systems, however, the DIN system ignores the 

 general form of the D-logio E curve. The DIN rating is based upon data taken in the 

 region of underexposure. The speed numbers are determined through the use of 

 developing technique which gives the greatest film sensitivity. 



Inertia-speed Ratings. — A considerable improvement over the threshold sensi- 

 tivity of specifying emulsion speed was made by Hurter and Driffield, who suggested 

 that the speed of a photographic material could be specified satisfactorily in terms of 

 its inertia. Thus they adopted as their criterion of speed the exposure indicated by 

 the extension of the straight-line portion of the D-log E curve to the axis of zero den- 

 sity, the corresponding exposure being the inertia of the photosensitive material. 

 By so doing, they took into account the shape of the H and D curve in the region of 

 correct exposure. The exposure Eh of Fig. 24, corresponding to the extension of the 

 linear portion of the D-\og E curve to the exposure axis, is called the "inertia" of the 

 emulsion, and the inertia value is frequently referred to as i. Hurter and Driffield 

 proposed to specify the speed or sensitivity S by the relation 



S =i (44) 



where i = the inertia of the photographic material Eh', 



k = an arbitrarily^ determined constant. 

 Hurter and Driffield originally chose the value 34 for k since, at the time they made 

 their researches, this figure gave speed values for practical materials which were 

 convenient in magnitude. However, other values for k have been used, the number 

 10 being in common use. 



Hurter and Driffield used the inertia as a measure of film speed since they took this 

 point to be indicative of the beginning of the region of correct exposure and proper 

 tone rendition. The H and D speed rating is, therefore, a system of numbers pro- 

 portional to the inertia values through the factor k. An essential difference between 

 the Hurter and Driffield system and the methods based on threshold speeds is that the 

 inertia is determined from measurements at several points on the curve, while in the 

 threshold or one-tenth-density systems, sensitivity is determined from a single observa- 

 tion. In this respect the H and D system is the more rational procedure since the 

 Scheiner, Eder-Hecht, and DIN systems ignore the linear portion of the curve. 

 However, the H and D system gives no indication of the latitude or exposure range of 

 the photographic material. 



Figure 25 shows how two emulsions could have the same threshold but different 

 inertia. Thus, according to the Scheiner system, both would have the same speed, 

 while, according to the H and D system, one emulsion would be much faster than 

 the other. On the other hand, the H and D system ignores completely the foot of 

 the curve. However, there are times when it is desirable to permit a portion of the 

 image to extend into this region. In the H and D system the relation of speed to 

 index number is linear, while in the Scheiner system it is not. Thus, 26° Scheiner 



