Hurter and Driffield

Ferdinand Hurter (1844–1898) and Vero Charles Driffield (1848–1915) were nineteenth-century photographic scientists who brought quantitative scientific practice to photography through the methods of sensitometry and densitometry.

Among their other innovations was a photographic exposure estimation device known as an actinograph.

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H & D curve used in sensitometry
Hurter & Driffield's actinograph .