Area measurements of apertures for exo-atmospheric solar irradiance for JPL

A comparison of the area measurements of the limiting apertures used for total solar irradiance measurements in the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) and Active Cavity Radiometer Irradiance Monitor III (ACRIM III) were conducted between the National Institute of Standards and Technology (NIST) and the Jet Propulsion Laboratory (JPL). The ACRIM apertures, due to their unique size and design, necessitated modifications to the NIST aperture measurement system. The changes and the validation procedures undertaken are described in this paper. This is part of an Earth Observing System (EOS)-sponsored international comparison of aperture area measurements of apertures that have institutional heritage with historical solar irradiance measurements.