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". Such devices were developed and described by Robert Hunt, secretrary of the
Royal Cornwall Polytechnic Societyin 1845, as an improvement on T. B. Jordan's 1839 "Heliograph". [Klaus Hentschel, "Mapping the Spectrum: Techniques of Visual Representation in Research and Teaching", Oxford University Press, 2002, ISBN 0198509537.]
Ferdinand Hurterand Vero Charles Driffieldpatented 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.
Pyranometer(a type of actinograph)
Hurter and Driffield
Wikimedia Foundation. 2010.