Single-grain-layer photographic plates were prepared from a non-color-sensitized, slow-speed, uniform-grain-size emulsion and exposed to very pure monochromatic light of wave-lengths λ3600A, λ4500A, and λ5000A. Intensity-scale characteristic curves were made on a double monochromator in which the energy of the exposures could be measured with an accuracy of ±5 percent. Data for plotting the characteristic curves were obtained by microscopic grain counts of developed and undeveloped grains for successive exposure steps. These were then plotted in several different ways: First, the fractions of developed grains, k/N (k is the developed and N the total number of grains per unit area) were plotted as a function of the incident exposure in terms of ergs per square centimeter of plate area. Second, the k/N values were plotted as a function of incident exposure in terms of quanta per average grain. Finally, using absorption data obtained for thin layers of silver bromide, the k/N values were plotted as a function of quanta absorbed per grain. Experimentally, it is found that the characteristic curves increase their slope with increasing wave-length of exposing radiation and also show a tendency to increase in slope with decreasing development time.
The characteristic curves are analyzed by means of the Poisson statistical equation to determine the number of quanta actually utilized in formation of the latent image. Preparatory to this analysis, exposures were made to alpha-particles, and it is shown that the single-grain-layer plates accurately obey the single-hit formula derived from the Poisson equation. From the analysis of the curves obtained with monochromatic light exposures, it is concluded that the threshold sensitivity of a grain, expressed as the minimum number of quanta to produce developability of the grain, is of the order of ten quanta per grain for the emulsion under test. The upper limit of sensitivity is specified by the number of quanta absorbed per grain as shown by the abscissa values of the characteristic curves.
Reciprocity-law failure measurements were made on the single-grain-layer plates, using unfiltered tungsten radiation. Time-scale characteristic curves were made over an intensity range of 1:16,384. The curves showed some loss of speed with reduced intensity but mainly a reduction in slope. This indicates that the grains of the upper and lower parts of the characteristic curve do not behave the same toward reduced intensity, and the grains of the upper part of the curve show the greater loss in speed at low intensity.
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