A combined optical diffuser and light concentrator uses a layer of optically transmissive material having a first surface to receive light at a range of angles of incidence and a second surface from which the light emerges at a range of angles of emergence, the second surface including a relief pattern...http://www.google.com.au/patents/US5861990?utm_source=gb-gplus-sharePatent US5861990 - Combined optical diffuser and light concentrator
