Actinometer

An actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time.

For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected.

His design used a blackened thermometer enclosed in a glass sphere to measure solar radiation, which he referred to as a "heliothermometer."

John Herschel further developed actinometers in the 19th century, including a design involving photochemical reactions to measure sunlight intensity, which was a significant step forward.

Herschel's actinometer involved observing the rate of a chemical reaction under sunlight, which allowed for more precise quantification of solar energy.