In the case where the surface between two bodies is perfectly polished, then the normal provided by the program is the normal to the surface defined by the body boundary. This is indicated by the the value POLISH=1 as returned by the GUPLSH function. When the value returned is < 1, then a random point is generated in a sphere of radius 1- POLISH, and the corresponding vector is added to the normal. This new normal is accepted if the reflected wave is still inside the original volume.