H and d curve
A graph in which the density of the photographic film (vertical axis) is plotted against the logarithm of the relative exposure (horizontal axis), to illustrate the photographic characteristics of an emulsion (e.g. on an x-ray film).
A characteristic curve depicts and measures the relationship between exposure and photographic density. It is used to describe the contrast characteristics of a photographic film for a particular range of exposure. The exposure needed to produce an image is also determined.
It is also called Hurter and Driffield curve (or H & D curve), named after Ferdinand Hurter and Vero Charles Driffield who first developed it.