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From George Eastman House : Notes On Photographs

The actinometer was a form of light-measuring device. It measured the actinic power of light by comparing the ongoing exposure of blue-sensitive photographic paper to a set of standards. These devices were most valuable when making prints by the carbon or gum methods, where visual progress of the print during exposure was impossible. The actinometer typically included a box containing a strip of sensitized silver chloride printing-out paper that was pulled into a window where it would be exposed to light.

A reference panel bearing the color of exposed silver chloride paper was positioned next to the window. Every time the sensitive paper printed out to the same degree as the standard, the photographer took note and pulled a new section of sensitive paper into position. By this method timing was always consistent and was based on the actual effect of light rather than on time.^[1]

1. ↑ Osterman, Mark. 2007. Actinometer. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 37, Focal Press.