SENSITOMETRY SINCE HURTER AND DRIFFIELD

THE best memorial to Hurter and Driffield is the continued application of their teaching by the photographic industry. The basis of photographic sensitometry is a characteristic curve of the material under test, and when we remember that the particular characteristic curve in use to-day is still known as the 'H and D curve', obtained by plotting density against the logarithm of the exposure, we realize the excellence of the work which was done so long ago by those two enthusiastic amateurs. The principle of sensitometry remains what it was when Hurter and Driffield left it. The details have changed; the values of the co-ordinates of the curve have been corrected and brought nearer to standardization ; new apparatus has been applied to the work and the results themselves are interpreted in new ways which make it possible to link sensitometry much more closely with practical photography. I propose to sketch in outline some of the ways in which the H and D curve has been pulled into shape in order the better to guide those who use photography and those who must 'provide the best kind of photographic material for each particular purpose.