## Abstract

In order to express numerically the speed of a photographic material it is necessary to choose a criterion upon which the computation may be based. Two criteria have been used quite generally: (a) the inertia, and (b) the exposure required to produce a specified (usually low) density. From both theoretical and practical considerations neither of these criteria are entirely satisfactory. The present paper discusses the possibility of using a minimum limiting gradient as a criterion of practical effective speeds, at least in the amateur field. This is not a new proposal but has not been used to any great extent because of the alleged difficulty of determining simply and precisely the exposure corresponding to a specified minimum gradient. A differential photometer, gradient photometer, is described which, when used with a sensitometric strip of proper characteristics, makes the determination of the minimum limiting gradient extremely simple and yields results of high precision. Data are given illustrating the agreement between different observers and the order of precision obtainable. It is not proposed to use a constant value of minimum gradient, but a value which is some function of the maximum gradient (gamma) to which the material is developed. No definite proposal as to what this function should be is made, but in order to illustrate the application of the method a tentative assumption is made that the criterion to be used in computing a value of practical speed is the exposure required to produce a gradient which is equal to one-half of maximum gradient (gamma). The superiority of this criterion over those in common use is discussed at some length. Illustrative data obtained by examination of some fifty different amateur negative materials are given which show the relation between speed numbers determined in this manner and those determined by the “inertia” and “fixed density” methods. The paper closes with suggested specifications for a sensitometer, gradient photometer, and technique for the standardization of this method.

© 1935 Optical Society of America

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