Corner Cubes
Corner Cube Retro-Reflectors have three mutually perpendicular surfaces and a hypotenuse face.  It operates on the principle of total internal reflection(TIR).  A beam entering the effective aperture is reflected by the three roof surfaces and emerges from the entrance/exit surface parallel to itself.  This property is independent of orientation of the retro-reflector, within acceptance angle limitations.  For applications in which either the acceptance angle for TIR is exceeded, or the reflecting surfaces can not be kept sufficiently clean for TIR, a metal or dielectric coating can be applied to the reflecting surfaces.