Direct Spectral Irradiance Measurements from Rotating Shadowband EKO Grating Spectroradiometer

In this paper, we explore a rotating shadow band (RSB) configuration for the EKO MS-711 spectroradiometerto measure the global, diffuse and direct components of spectral irradiance. An RSB configuration allows lowering the costs associated with the instrumentation and maintenance required to measure the components of spectral irradiance. Since only one spectroradiometeris used for the measurement of the spectral irradiance components, discrepancies associated to sensor calibration can be minimized. This work presents a study to validate the EKO RSB spectroradiometeraccuracy in measuring the direct normal irradiance (DNI). A comparison is made between the measurements performed with the RSB spectroradiometerand a collimated spectroradiometerin Mauna Loa Observatory. The results of the comparison show an agreement within 2% to 5% between the DNI estimated by the RSB and collimationconfigurations for solar zenith anglessmaller than 70 °. Larger deviations approximated to 10% are found for larger angles of incidence.