From AMS Glossary
The instrument consists of a radiation sensing element enclosed in a casing that is closed except for a small aperture through which the direct solar rays enter. Pyrheliometers can be classified on the basis of the sensing elements employed. In one form the sensing element is a blackened water calorimeter. The rise in the temperature of the water gives a measure of the amount of radiant energy absorbed during the exposure of the instrument. Another type of sensing element consists of a blackened plate of high heat capacity. When radiation is allowed to fall on the plate for a period short compared to the thermal time constant, the temperature rise of the plate is proportional to the intensity of the incoming radiation. A third type of sensing element consists of a pair of plates, one blackened and one reflecting, that are continuously exposed to the incoming radiation. The temperature differential between the plates is proportional to the intensity of the incoming radiation. See Hand (1946) for descriptions of various types of pyrheliometers, for example, silver- disc pyrheliometer, water-flow pyrheliometer, Eppley pyrheliometer, spectropyrheliometer, Michaelson actinograph.
Hand, I. F. 1946. Pyrheliometers and Pyrheliometric Measurements. U.S. Weather Bureau.