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 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 8 × 10−4 and 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.

Characterization of absolute cavity radiometers for traceability to SI of solar irradiance

“…However, ACR versions adapted for ground-level operation have rarely been characterized, especially for the commercial ACR versions, and even less frequently by independent laboratories [58][59][60]. In practice, it is very difficult for a laboratory not involved in the design or engineering of an ACR to face the task of its characterization.…”