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SENSITOMETRY
113
After the development of the exposed plate the next necessary step involved the measurement of the density which resulted from the exposure and development. For that purpose Hurter and Driffield devised another instrument which was in reality a grease spot photometer. Figure II shows a sketch of the small chamber containing the grease spot and also the position of the two comparison light sources SI and S2. The grease spot is inserted through a slit in the upper side of the chamber in the center of the instrument and its position is shown in the figure. The box containing the grease spot slides along a track and is fitted with an eye piece through which light from the two sources can be viewed as two distinctly different fields. The density to be measured is placed over one source of light and the grease spot chamber moved toward that side of the instrument until a balance is obtained by the two brightnesses. By proper calibration it was possible with this instrument to make readings in terms of density. It might be well to state here that density is defined as follows: D=log10 O=log]0 J where O represents opacity and T transmission. I0
Grease Spot
Fig. 2
With the completion of the measurements of the densities on the test strip the next step was to construct a curve showing the relationship between density and the logarithms of exposure which cause these densities. Figure III shows a curve typical of the type obtained by Hurter and Driffield. The sector wheel, as previously stated, contained nine openings, which increased by a ratio of 2. In other words, the exposures increased in that ratio. As above defined, density is a logarithmic function. Therefore, it was advisable to plot the logarithm of the exposures inasmuch as it would be difficult to plot strips covering the exposure range in straight exposure units. The following tabulation shows the relative exposure values produced by the sector wheel together with the logarithmic values of these exposures.