Actinograph

An actinograph is an instrument for measuring or estimating the amount of light available, in terms of its ability to expose photographic film.

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.

Such devices were developed and described by Robert Hunt, secretary of the Royal Cornwall Polytechnic Society in 1845, as an improvement on T. B. Jordan's 1839 Heliograph.

These were slide rules, not measuring instruments, and did not produce a graph, but Hurter and Driffield adopted the same name for it.

(Fellow of the Royal Meteorological Society and Principal of the Royal Albert Memorial University College of Exeter) developed a version of an actinograph for meteorologists, to observe and record the change of radiation.