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Actinometry is the measurement of the intensity of radiation - particularly light - and especially by measuring the speed of a photo-chemical reaction.
Some early light meters (often called actinometers) worked by timing colour changes, caused by the chemical reaction to the light on strips of chemical-infused paper - a short piece of which could be uncovered from a dark box, allowing it into the light for use. The Watkins Bee meter is an example of such an actinometer.