An optically retro-reflecting sphere includes an inner sphere and an outer concentric spherical shell, and can be coupled to a reflective surface. Light entering the optically retro-reflecting sphere from an incident direction is reflected by the sphere generally towards an exit direction, where the exit direction is parallel and opposite to the incident direction. The inner sphere and outer shell have different refractive indices, selected so that the reflected light is reflected with a non-uniform angular distribution about the exit direction.