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. Herschel's version was influential and helped standardize measurements of solar energy. Herschel introduced the term actinometer, the first of many uses of the prefix actin for scientific instruments, effects, and processes.

The actinograph