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Abstract
The International Pyrheliometric Scale of 1956 (IPS 1956) is based upon the original standards of the Ångström and Smithsonian scales by suitable corrections (+ 1.5 and −2.0%, respectively). The old standards will soon be replaced by new kinds of absolute actinometers, and a revision of the international scale is at hand. The present paper reviews the Compensation Principle with special regard to absolute pyrheliometry and the increased demands of accuracy since the principle was formulated by Knut Ångström in 1893. The Swedish manufacturers have maintained a practice of measuring the characteristics of self-calibration of newly-made pyrheliometers so as to provide a check of the maintenance of the Ångström Scale.