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 photo-Speeds by metric method (using a wedgd 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 incorp rating 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.
CLIFTON TUTTLE, "Methods and Instruments for the Determination of Photographic Speeds by Measurement of Relative Characteristic Gradients," J. Opt. Soc. Am. 29, 267-273 (1939)