An instrument is described which has been built for the automatic recording of sequence exposures on 4-by-10-in. photographic plates. Twelve shutter slides are independently timed so that on one plate up to 384 4-by-5-mm rectangles can be uniformly exposed to high and low intensity radiation. The present design is for an intensity ratio of 100:1, where both intensities are below optimum. With minor modifications the instrument can be converted to operate at other intensity ratios. The densitometer readings are checked from calibration strip data placed adjacent to the sequence exposures. The corrected data, following conversion to Seidel values, are plotted as families of characteristic curves, with the sequence exposure ratio as the constant parameter. Isodense loop data are read from the curves at the intersection with a line drawn parallel to the exposure axis.
A theoretical discussion of isodense loops is presented which is based on the hypothesis that for isodense exposures the probability of rendering the last grains developable must be equal. It is shown how this method should yield the limiting slope of the low intensity failure curve and the minimum number of quanta absorbed by the average grain to become developable. Preliminary results presented here for Eastman Kodak Emulsion Type 33 are within expectation.
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