| Description: The incidence_angle element provides a measure of the lighting condition at the intercept point. Specifically, it is the angle, measured at the intercept point, between a vector in the direction of an incoming photon from the illumination source and the local surface normal vector. The incidence_angle varies from 0 degrees when the intercept point coincides with the subsolar point to 90 degrees when the intercept is tangent to the surface of the target body. In the context of planetary rings, the local surface normal vector is always on the side of the ring plane illuminated by the light source. Thus, the incidence_angle varies from 0 degrees if the light source illuminates the rings face-on, to 90 degrees if the light source illuminates the rings edge-on. |