Normal Radiation

radiation striking a surface that is facing the sun. Mathematically, the wordnormal is the vector (direction) that is perpendicular to a surface, andthe direction of a normal radiation source is perpendicular to a radiationsource. Global (total) normal solar irradiance is all radiation that strikes a flatsurface that faces the sun, whiledirect normal solar irradiance excludesall radiation that does not come from the direction of the sun in the sky.