title: Optical reflectance of pyrheliometer absorption cavities: progress toward SI-traceable measurements of solar irradiance.
abstract: 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 a normal incidence as a function of the viewing angle and position on the cavity cone. By numerically integrating the measured angle-resolved scatter over both the direction and position and accounting for an obstructed view of the cavity, we determined that the effective cavity reflectance was between 8×10-4and 9×10-4. Thus, the absorptance of the three cavities ranged from 0.99909±0.00014 to 0.99922±0.00012 (k=2 combined expanded uncertainties). These measurements, when extended over the spectral range of operation of the pyrheliometer, are required to establish SI traceability for absolute solar irradiance measurements.
authors.forename: Heather J
authors.initials: HJ
authors.lastname: Patrick
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Germer
authors.forename: Clarence J
authors.initials: CJ
authors.lastname: Zarobila
authors.forename: Catherine C
authors.initials: CC
authors.lastname: Cooksey
authors.forename: Howard W
authors.initials: HW
authors.lastname: Yoon