PHOTOGRAPHIC SENSITOMETRY 177 



The instrument shown in Fig. 16 is usually referred to as the Marten's photometer 

 head, and in order to construct a satisfactory instrument for the measurement of 

 density, this must be associated with certain elements suitable for illuminating the 

 photographic density to be measured and for providing the comparison beam. One 

 arrangement for the provision of satisfactory illumination is shown in Fig. 16, in which 

 M represents a ground-glass diffusing surface illuminated to a relatively high level by 

 some external light source placed to the left of M in the figure. The total-reflecting 

 prism p reflects light from M through one of the apertures in the nosepiece of the 

 photometer head, thus serving to provide the comparison beam which illuminates 

 one-half the photometric field. A lens I, mounted as shown just below the nosepiece 

 of the photometer head, forms an image of M approximately in the plane occupied by 

 the apex of the Fresnel biprism. A second total-reflecting prism g reflects light from 

 M through the other aperture of the nosepiece, thus illuminating the other half of the 

 photometer field. The photographic plate or film to be measured is placed in the 

 position as indicated at P. In this arrangement of the Marten's polarization photom- 

 eter, the illumination of the photographic deposit to be measured is by means of a 

 semispecular beam of Hght; hence the value of density approaches that of specular 

 density for the deposit in question. By placing a small disk of white pot-opal glass 

 immediately below P, it will be possible to obtain readings of diffuse density. In this 

 case, it will, of course, be necessary to balance the illumination by the insertion of a 

 proper amount of absorbing material in the comparison beam reflected by the small 

 prism p. Under such conditions, it is somewhat difficult to obtain sufficient illumina- 

 tion to read very high densities with precision. The ground glass M may, however, 

 be removed, and by using a light source of high intrinsic brilliancy and a properly 

 designed optical system, a high concentration of light flux may be obtained on the 

 opal glass directly underneath P; in this way more satisfactory results may be obtained 

 in reading high values of diffuse density. 



Interpretation of Results. — It now remains to interpret the results of the exposure, 

 development, and density-determining processes which we have gone through for our 

 sensitometric determinations. There are a number of ways in which the results of the 

 sensitometric process may be interpreted, but generally graphical methods of inter- 

 pretation are most useful and direct. 



H and D Characteristic. — The most important relation in photography, so far as 

 concerns the characteristics of photosensitive materials, is that showing the relation 

 between the exposure of the material and the resulting density of the silver deposit. 

 This relation is shown in Fig. 17 by means of the familiar characteristic curve or H 

 and D curves, named in honor of Hurter and Driffield — early research workers in the 

 field of photographic sensitometry. This characteristic curve is obtained by giving 

 the photographic-sensitive material a series of graded exposures, the exposure of one 

 step bearing a known relation to that of the preceding and succeeding steps. The 

 density of each step in the developed silver image is then measured and is plotted 

 against the logarithms (to the base 10) of the corresponding exposures. Because of 

 the scales used, these characteristic curves are sometimes referred to as the Z)-logio E 

 curves. One reason for using a logarithmic scale for exposure is because large ranges 

 of exposure values are encountered, and this wide range could not be compressed con- 

 veniently into a linear scale. By plotting the density against the logarithm of the 

 exposure, it is found that the characteristic curve begins by curving upward from the 

 zero-density axis. An approximately linear region of the curve then usually fol- 

 lows, especially in the case of negative materials. The curve finally decreases in 

 slope and, after reaching some maximum value, generally decreases in density for 

 extremely large values of exposure. These five regions are commonly known, respec- 

 tively, as the region of no exposure (AB), the region of under exposure (BC), the 



