Active cavity radiometric scale, international pyrheliometric scale, and solar constant

The active cavity radiometer type II, a new and accurate standard detector, has been developed for the absolute measurement of optical radiant flux. The active cavity radiometric scale (ACRS), defined by the active cavity radiometer (ACR), and the international pyrheliometric scale (IPS), defined by a U.S. standard angstrom pyrheliometer, have been compared in recent experiments. Simultaneous measurements of solar irradiance demonstrated an average systematic difference between the two scales of 2.2%, the measurements on the ACRS exceeding those on the IPS. An analytical study of the sensitivity of the ACR to sources of experimental error is presented. The uncertainty in the ACRS is found to be less than ±0.5% at the one solar constant level relative to the absolute scale based on fundamental physical principles. In August 1968 two ACR's measured the solar irradiance at an altitude of 25 km in a balloon-flight experiment. The solar-constant value derived from this measurement was Ho = 137.0 mw/cm2.