192 HANDBOOK OF PHOTOGRAPHY 



The Scheiner ratings are derived from the Scheiner type of sensitometer which 

 employs a sector wheel having a continuously graded, rather than a stepped, exposure 

 variation. The aperture of the original Scheiner sensitometer was bounded by a 

 continuous logarithmic curve arranged so that the maximum aperture was 100 times 

 as large as the minimum. A linear scale, divided from 1 to 20 in equally spaced 

 imits, was placed along the slot aperture, so that the 20 sensitometric steps corre- 

 sponded to a sensitivity or speed range of 100 to 1. This scale was placed between 

 the rotating sector of the sensitometer and the photosensitive material, so that the 

 images of reference cross lines, together with the Scheiner numbers, appeared on the 

 plate after exposure and development of the sensitometric strip. Because of this 

 arrangement the Scheiner speed scale consists of numbers in arithmetic progression 

 from 1 to 20, corresponding to a speed or sensitivity range of from 1 to 100. In the 

 Scheiner type of sensitometer the larger numbers represent denser deposits of silver 

 produced during exposure. The speed or sensitivity of the material was expressed 

 by the densest line or number which could be read. The relative sensitivity given 

 by any specified number in the Scheiner rating is 1.27 times as great as the sensitivity 

 represented by the next lower number in the Scheiner scale. 



The Austrian Eder-Hecht speed is similar to the German Scheiner rating in that an 

 arithmetic progression of the speed numbers represents a geometric progression of 

 relative sensitivity or speed. The Eder-Hecht sensitometer consists of a continuously 

 graduated density wedge, behind which the photosensitive material is placed for an 

 exposure of specified intensity and duration. Across this wedge, lines representing 

 density contours are ruled. The lines are numbered so that the greatest density of 

 the Eder-Hecht wedge is represented by the largest numbers. The density gradient 

 of the Eder-Hecht wedge is stated by Jones to be 0.4 density units per centimeter but 

 the tablets are not very precise and discrepancies between wedge calibration is fre- 

 quent. If it is assumed that the wedge has a uniform density gradient, an arithmetic 

 progression of speed numbers represents a geometric progression of relative sensitivity 

 or speed. In this respect, at least, the Scheiner and Eder-Hecht ratings are similar. 

 However, the actual numbers used to designate the sensitivity of a particular film in 

 the two systems are considerably at variance. 



Although the method of sensitometry which makes use of the minimum detectable 

 deposit of density is simple and requires no sensitometric measuring equipment, the 

 system is not a very satisfactory one and has several inherent and serious disadvan- 

 tages. The main objection to this system is that it is difficult to determine the 

 exposure for which a "just detectable" density is produced. Even if it were not 

 difficult to determine minimum detectable density, the shape of the D-logio E curve is 

 such that the point Es of Fig. 24 cannot be determined accurately. Furthermore the 

 speed rating is determined from the characteristics of the Z)-logio E curve at the foot 

 of the curve corresponding to underexposure. The ratings based on minimum 

 detectable density differences consequently do not give any information for the region 

 of correct exposure, nor do they give any information relating to the extreme upper 

 portions of the H and D curve. It is perfectly possible, therefore, that the minimum 

 detectable density may be due to fog rather than intentional exposure. It is also 

 possible for two emulsions having widely varying Z)-logio E curves to produce fog 

 density at the same minimum exposure. Thus, whereas the two films might bear the 

 same Scheiner or Eder-Hecht ratings, their D-logio E characteristics could easily be 

 quite dissimilar. The Scheiner rating system has found extensive use, especially in 

 Europe, despite its obvious disadvantages. 



A modification of the threshold-sensitivity method which overcomes the difficulty 

 of determining the minimum detectable density is the Deutsche Industrial Normal 

 rating system. This is usually referred to as the DIN system. According to the 



