PHOTOGRAPHIC SENSITOMETRY 185 



vik^, makes use of a disk so cut as to produce 12 exposures, each of which is twice the 

 duration of the preceding one. The total range of exposures is 2048 to 1 for a com- 

 plete rotation of this disk. The absolute values of exposure are determined by the 

 velocity with which the disk rotates and the intensity of the energy impinging upon 

 the plate. The radiant energy acting upon the photographic plate is measured by 

 means of a thermopile placed in the same plane as that of the material being exposed. 



Except for the radiation-dispersion system and the greater sensitivity and refine- 

 ments necessary because monochromatic rather than heterochromatic radiations 

 are being measured, the monochromatic sensitometer is similar to the ordinary 

 sensitometer; the exposed test strips obtained are similar to those of the ordinary type 

 of sensitometer. The exposed strip is developed under standardized conditions, and 

 the densities corresponding to the various exposure conditions are determined in the 

 manner already described. Where it is necessary to study the effects of developing 

 conditions, a series of strips is made under identical exposure conditions, but the 

 conditions of development are altered as may be required. With a series of such 

 strips, representing various conditions of exposure and development at any given 

 monochromatic radiation, the characteristics of the photographic material for that 

 radiation may be determined. By making other observations at different wave- 

 lengths, the complete wavelength-exposure-density characteristics of the material 

 may be determined. The results may be plotted in the usual manner as a family of 

 Z)-logio E characteristics or as gamma-development time curves except for the manner 

 in which the radiant energy of exposure is specified. 



In the ordinary methods of sensitometry, exposure values are given in terms of 

 the meter-candle-second for a white light source. A white light source has no signifi- 

 cance in monochromatic sensitometry, so that some other unit of measurement is 

 needed. The unit of exposure which has been most widely adapted for this purpose is 

 the erg. Since the photographic material integrates more or less perfectly the energy 

 which falls upon it over a period of time, it is necessary, of course, to include the time 

 factor, and in expressing photographic exposure in energy units it is necessary to 

 multiply the rate at which energy falls upon the surface (radiant flux density) by the 

 time during which the exposure persists. Exposure, therefore, must be expressed in 

 terms of ergs (or other suitable energy units) per unit area. 



The monochromatic sensitometric data may be plotted in the usual manner except 

 that curves for each monochromatic radiation will be obtained instead of a single set 

 of data corresponding to white light radiation. If several H and D curves are 

 plotted for varying wavelengths of monochromatic radiation, it will be found that the 

 Z)-logio E curves vary, depending upon the wavelength at which measurements were 

 determined. If families of D-logio E curves are available, the characteristics of the 

 photographic material may be plotted as a function of the wavelength of radiation. 

 The curve of Fig. 20 shows the gamma versus wavelength plot for varying develop- 

 ment conditions, whereas Fig. 21 shows a family of H and D curves, one curve being 

 determined for each of several wavelengths of monochromatic radiation. 



With a set of data available showing density and gamma as a function of exposure, 

 developing conditions, and various monochromatic radiations, it is possible to deter- 

 mine the relative spectral sensitivity of the material. Several possible methods are 

 available for expressing spectral sensitivity. We might, for example, compute spectral 

 sensitivity in terms of the reciprocal inertia for various monochromatic radiation 

 measurements in much the same way as sensitivity for a white light source is specified 

 in the H and D system. We might define spectral sensitivity in terms of the energy 

 required to give a specified value of density (say unity) for a fixed time of development, 



■ Jones, L. A., and O. Sandvik, Spectral Distribution of Sensitivity of Photographic Materials, 

 J. Optical Soe. Am., 12, 401, 484 (1926). 



