Partitioning solar radiation into direct and diffuse, visible and near-infrared components

Measurements of total and diffuse, solar and visible (photosynthetically active, PAR) irradiance were made during portions of the summer months of 1979, 1980, and 1981 near Scottsbluff, NE. The objective of this experiment was to obtain equations to predict direct beam and diffuse irradiance in both the visible (PAR) and near-infrared wave bands from measurements of only the total incoming solar radiation. Relationships for potential values of direct and diffuse, visible and near-infrared radiation were established from “clear day” experimental data. Estimates of total visible (PAR) and near-infrared radiation were made by partitioning the measurement of solar radiation using estimates of potential visible and potential near-infrared radiation. Estimates of direct components in the visible and near-infrared were made with relationships developed from the experimental data. There was good agreement between predicted and measured values of the various components of solar radiation. In addition, a correction term for the measurement of diffuse solar radiation with a silicon cell pyranometer was developed.