CORNER CUBE prism (Retroreflector ; TRIHEDRAL prism) In the corner cube prism three reflecting surfaces are erpendicular each to the other (like side walls of a cube). The input surface is perpendicular to the cube diagonal. Disregarding the angle of incidence the output beam is parallel to the input one, but it is of opposite direction. In set of several corner prisms, for better matching, their entrance – exit surface is often hexagonal. In case of this prisms the phenomenon of full internal reflection occurs so it is not necessary, (however it is possible) to put mirror coating on the reflecting faces.