An anisotropic shadow ring correction method for the horizontal diffuse irradiance measurements

Abstract

Diffuse irradiance is usually measured with a shadow ring which prevents beam radiation from entering the measuring instrument. The shadow ring also obscures part of the sky, and therefore, it is necessary to correct the measured diffuse irradiance. This correction is often assumed to be constant at a certain time of the year and at a certain latitude. However, the radiance distribution of the sky is not isotropic, and the anisotropy must be taken into account in the correction. In this paper, a new numerical method for calculating the shadow ring correction is presented. The correction factor is calculated directly from the radiance distribution of the sky. For Helsinki latitude (60°N), the yearly average isotropic correction factor is 1.10 and the additional anisotropic correction factor is about 1.05. Realistic sky models, e.g. Perez all-weather model, were used in the calculation.