Angle factor [Mean angular radiation coefficient, Mean angle coefficient]
Ratio of the radiant flux leaving the surface of one body and reaching the surface of another to the total natural radiant flux issuing from the entire surface of the first body in all possible directions within a hemispherical solid angle.
Comment. It is assumed that the radiating body (area element) has the same intensity at all points and in all directions and that the separating medium is transparent.
Heat & Mass Transfer, and Fluids Engineering