ABSTRACT
The direct solar radiation passing through the Earth’s atmosphere is absorbed and/or scattered by molecules, aerosols, water vapor and other gases, and clouds. Alternatively, diffuse horizontal irradiance can be considered as the skylight portion of the total hemispheric, or global horizontal irradiance, remaining after removing the direct normal irradiance (DNI). Diffuse irradiance measurements have become more sophisticated with the development of automated tracking devices that dependably keep the direct solar radiation from reaching the detector of the pyranometer. To align the fixed shade, the plane of the arc formed by the shadowband or the shade ring should be perpendicular to the polar axis. The preferred, but labor-intensive, method for calibrating any pyranometer is to alternately shade and unshade the pyranometer while measuring the reference DNI with an absolute cavity radiometer. The Sun appears yellow or red near the horizon since Rayleigh scattering preferentially removes the bluest light from the transmitted DNI.
