The researches of Ferdinand Hurter (1844-1898) and Vero Charles Driffield (1848-1915) in the 1880s and 1890s established the basic principles of densitometry and sensitrometry that they applied to photographic exposure measurement. Their work was based on extensive observation and experimentation and was the first attempt to systematically relate light intensity and the density of the photographic plate. It was not the first attempt to produce a method of determining exposure by calculation or measurement but it was not until commercial manufacturers could produce photographic plates of consistent sensitivity to a widely adopted standard that exposure measurement devices could become widespread.
Formal measurements were first conducted and published by Bunsen and Roscoe in 1858 which connected sunlight with the position of the sun to time of day and year. This work was expanded and developed by Hurter and Driffield who published extensive tables in 1888. Their work produced a H & D number that was used to indicate sensitivity and crucially they showed that each dry plate could be allocated a number which could form the basis of an exposure calculation.
The commercial outcome of this work was their Actinograph, a calculator, which was patented in 1888 (British patent number 5545) and initially sold from 1889 from Driffield's house in Widnes by mail order. In October 1891 Frank Bishop of Marion and Co. wrote to Driffield proposing to adopt the H & D scale on each packet of plates sold by them and requesting terms to sell the Actinograph. It was sold from 1892 by Marion & Co. for a range of different latitudes and longitudes and Marions adoption of the system on their plates encouraged wider international use. A second model of the Actinograph based exclusively on slide rules was introduced in 1898.
The patent described the Actinograph which 'consists of a slide-rule, two scales of which are fixed and the other two movable. The fixed scales are subdivided with divisions corresponding to the logarithms of numbers which indicate according to suitable units, the one the intensity of light for the given latitude, time of the year, and time of the day, and the other the 'inertia' or inverse of the sensitiveness of the plate. The movable scales are subdivided with divisions corresponding to logarithms of numbers which indicate according to suitable units, one the speed of the lens, and the other the length of the exposure. The length of exposure will thus be equal to the intensity of the light multiplied by the speed of the lens and divided by the 'inertia' of the plate. This can be shown by the ordinary manipulations of the slide-rule'.