Corner cube prisms

Corner cube prisms are prisms which act as corner reflectors, a type of retroreflectors. The basic operation principle is that there are internal reflections on three mutually orthogonal prism surfaces, leading to a direction of a reflected beam which is nominally parallel to the direction of the incident beam — with the accuracy limited only by the accuracy of the surface orientation of the prism. Precision prisms can offer excellent parallelism of incoming and reflecting beams. It is usually specified as an angular deviation, e.g. < 3 arcseconds.