We have accurately determined the absorptance of three pyrheliometer cavities at 532 nm by measuring the residual reflectance using an angle-resolved bidirectional reflectometer. Measurements were performed at normal incidence as a function of viewing angle and position on the cavity cone. By numerically integrating the measured angle resolved scatter (ARS) over both direction and position and accounting for an obstructed view of the cavity, we determined that the effective cavity reflectance was between 0.08 % and 0.09 %. Thus, the absorbtance of the three cavities ranged from 99.909 (0.014) % to 99.922 (0.012) % where the value in parentheses indicates the k=2 expanded uncertainty. These measurements, when extended over the spectral range of operation of the pyrheliometer, are required to establish traceability for absolute solar irradiance measurements.
reflectance, scattering, solar, irradiance, BRDF, goniometer, directional-hemispherical reflectance, diffuse reflectance