Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements

TLDR

The multifilter rotating shadow‑band radiometer is a ground‑based instrument that uses interference‑filter photodiodes and an automated rotating shadow‑band technique to obtain spectrally resolved measurements of direct‑normal, total‑horizontal, and diffuse‑horizontal irradiances at seven wavelengths between 350 nm and 1.7 µm. It measures all three irradiance components with the same detector at each wavelength, and the automated shadow‑band technique guarantees identical calibration coefficients, while direct‑normal observations enable Langley analysis for optical depth retrieval and ongoing calibration against the solar constant. The instrument achieves direct‑normal spectral irradiance accuracy comparable to tracking radiometers and outperforms conventional instruments for diffuse and total‑horizontal irradiances due to its near‑cosine angular acceptance, with long‑term stability tied to the solar constant through routine data analysis.

Abstract

The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter–photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 μm) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees that the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.