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...http://www.google.fr/patents/US7224533?utm_source=gb-gplus-shareBrevet US7224533 - Optically retro-reflecting sphere
