actinometer (ăkˌtənŏmˈətər) [key], instrument used to measure the heating power of radiation. Actinometers are used chiefly in meteorology to measure solar radiation as transmitted directly by the sun, scattered by the atmosphere, or reflected by the earth. A number of different designs have been developed, including a small bimetallic strip or two bimetallic strips, one black and one white, heated by radiation. The subsequent bending of the strips is measured. Knowledge of such factors as the heat capacity of the strips, their surface area, their reflectivity, and the differences in their curvature produced by a given change in temperature gives the heating power of the radiation. In other designs, a sheet of photographic paper is exposed to the radiation to provide a measure of the total radiation over a period of time; cadmium sulfide photoconductive cells are also used, in which differences in light intensity change the resistance of a circuit.