It has been proposed that the speed of a photographic emulsion be specified upon the basis of the exposure value at which the gradient of the H and D characteristic becomes equal to a definite fraction of the average gradient throughout the tonal range used in making the camera exposure. The adoption of such a criterion necessitates the use of easily applied methods and special instruments for the determination of gradients and gradient ratios. In this paper a number of such methods and instruments are discussed under the following headings: (1) The graphical method. (2) A device for reading speed directly from the plotted characteristic curve. (3) A direct visual photometric method (using a wedged sensitometric strip). (4) An alternative direct visual method (using a triple sensitometric wedge and a special photometric field). (5) A direct photoelectric method of measuring gradient (which reduces the error in gradient determination occurring in visual photometry). (6) A photoelectric method incorporating a constant gradient ratio device (which eliminates the need of mental calculation). (7) Making automatic the determination of speed (pointing out means for automatic recording of results). Errors introduced by non-uniformity of coating and processing and means for minimizing these errors are discussed.
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