An optical device includes four prisms joined into one body to combine light beams, a first type of multilayer coating being provided between joining surfaces of the four prisms, and a second type of multilayer coating being provided on at least one non-joining surface of the four right-angle prisms....http://www.google.com.au/patents/US6155685?utm_source=gb-gplus-sharePatent US6155685 - Projector
