Vero Charles Driffield (1844-1898) and Ferdinand Hurter (1848-1915)

hurterFerdinand 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.

Through a common interest in music Hurter aand Driffield became friends while both worked at the Gaskell-Deacon Works in Widnes, Lancashire. Around 1876 Driffield persuaded Hurter to take up photography as a hobby. Hurter applied his scientific mind to photography and together they carried out important research into the subject. They published eight papers and in 1898 they were jointly awarded the Progress Medal of the Royal Photographic Society

driffieldAmong their innovations was a photographic exposure estimation device known as an actinograph An actinograph is an instrument for measuring or estimating the amount of light available, in terms of its ability to expose photographic film. That is, it measures the actinic or chemical intensity of light, as opposed to radiometric or photometric amount of light.

The earliest actinographs were 24-hour recording devices, using a rotating cylinder of photographic paper exposed through a wedged-shaped slit to record a graph of actinic light during the period of a day; hence the graph suffix in actinograph.

In 1888, Hurter and Driffield patented a device for estimating the actinic power of sunlight and for computing exposure times and apertures for cameras, based on the plate speed, time of day, time of year, and latitude. These were slide rules, not measuring instruments, and did not produce a graph, but Hurter and Driffield adopted the same name for it.

They also created a practical system for measuring the sensitivity of an emulsion. In their system, speed numbers were inversely proportional to the exposure required. For example, an emulsion rated at 250 H&D would require ten times the exposure of an emulsion rated at 2500 H&D.

The methods to determine the sensitivity were later modified in 1925 (in regard to the light source used) and in 1928 (regarding light source, developer and proportional factor)—this later variant was sometimes called “H&D 10”. The H&D system was officially accepted as a standard in the former Soviet Union from 1928 until September 1951, when it was superseded by GOST 2817-50.