On the Ångström and Smithsonian absolute pyrheliometric scales and the International Pyrheliometric Scale 1956

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Abstract
The generally accepted mean difference between measurements of solar irradiance by means of an Abbot silver-disk pyrheliometer, referenced to the Smithsonian scale of 1913, and by means of an Ångström electric-compensation pyrhel ometer, referenced to the Ångström scale of 1905, is 3.5%, measurements on the Ångström scale being lower. In September of 1956 at Davos the International Radiation Commission of the IUGG recommended the adoption of a new scale to be known as the International Pyrheliometric Scale 1956 (IPS 1956). To express radiation observations on the IPS 1956, measurements made on the Ångström scale of 1905 were increased by 1.5%, while measurements made on the Smithsonian scale of 1913 were decreased by 2.0%. A comparison of the Ångström scale of 1905 with the Smithsonian scale of 1913 at Toronto from 1955 to 1957 yielded a mean difference of 4.6% between them, which is 1.1% greater than the original 3.5% difference upon which the IPS 1956 was based. During the Second International Comparison of Pyrheliometers at Davos in 1964 the mean difference between the Ångström scale of 1905 and the Smithsonian scale of 1913 was found to be 4.85%, and during the Third International Comparison in 1970 to be 4.4%. Measurements made with a Kendall cavity-type self-calibrating pyrheliometer during the 1970 comparisons, however, agreed within 0.2% of the Smithsonian scale of 1913 reduced by 2.0%. A new practical scale for use as reference for meteorological radiation measurements is urgently required as a replacement for the IPS 1956. Such a scale can now be realized with high accuracy by means of a radiometer incorporating a cavity-type receiver and an aperture with carefully defined geometry. DOI: 10.1111/j.2153-3490.1973.tb00642.x

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