A radiation scale for measurement of solar exitance (irradiance). Prior to 1956, the Ångström Scale (ÅS) (1905) and Smithsonian Scale (SS) (1913) were used. Each scale was calibrated against a different radiation detector (i. E. , the Ångström compensation pyrheliometer and water-stirred pyrheliometer, respectively), and yielded slightly different values for the irradiance, with the ÅS reading roughly 3. 5% lower than the SS. The International Pyrheliometric Scale (IPS), defined in 1956, represented a numerical compromise between these two scales. In 1975, the IPS was replaced by the Absolute Radiation Scale (ARS). The ARS is calibrated against six absolute cavity radiometers maintained at the World Radiation Center in Davos, Switzerland. The variation among the six radiometers is about 0. 3%. The IPS was found to give measured irradiance levels that were about 2%–3% percent lower than the more precise ARS.