n (ca. 1965) A retroreflective surface or device is one which reflects in incident ray of light back along the direction of incidence with only limited spread, without requiring the incident ray to be normal to the surface. Such a surface or device differs from both a perfect diffuser and a perfect mirror. It may utilize small triple-reflection cube corners, small transparent spheroids, multiple hemispheric indentations, a multiple lens sheet backed by a reflective surface, or some simple derive. See Reflex Reflector.