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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^-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.

SUBMITTER: Patrick HJ

PROVIDER: S-EPMC5109930 | biostudies-other | 2016 Aug

REPOSITORIES: biostudies-other

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