Each of a plurality of Fresnel prisms 12 is formed so that a refracting surface 12 thereof includes a non-light incidence surface 12c upon which any ray of light emitted from a projector 1 is not directly incident because it is intercepted by another Fresnel prism 12, the non-light incidence surface...http://www.google.de/patents/US7242536?utm_source=gb-gplus-sharePatent US7242536 - Fresnel optical element and projection display device
