A retroilluminating indirect gonioprism comprising an optical prism, an oblique reflector and light baffle to reduce the intensity of light reflected from the gonioprism anterior surface back toward an observer, and an opaque surface to substantially prevent retinal burns when using the gonioprism with...http://www.google.de/patents/US5537164?utm_source=gb-gplus-sharePatent US5537164 - Retroilluminating indirect gonioprism
