1. Direct normal irradiation is the amount of direct solar irradiance that is received per unit area by a surface which is perpendicular to the sun’s rays.
2. Direct irradiance is the quotient of the radiant flux on a given plane receiver surface received from a small solid angle centered on the sun’s disk to the area of that surface. If the plane is perpendicular to the axis of the solid angle, direct normal solar irradiance is received
Solar radiation, methodology, modeling solar radiation, solar resource
http://www.3tier.com/en/support/solar-online-tools/what-direct-normal-irradiance-solar-prospecting/ (1.); http://www.sciencedirect.com/science/article/pii/S0038092X14004824 (2.)
Author: Michael Schaper