A model to determine the instantaneous transmittance of glazing for specific direct and diffuse solar radiation conditions is developed. Plots illustrative of the generic behavior of the mean monthly hourly transmittance of glazings in different planes are presented. Using test reference year meteorological data for 29 western European locations, limiting values are determined for clearness indices, orientations, and slopes for which an isotropic diffuse assumption is valid for calculating mean daily transmittance.