An overhead projector comprises, a main body, a light source disposed in the main body for emitting a light, a Fresnel lens disposed on an upper surface of the main body, an internal mirror for reflecting the emitted light toward the Fresnel lens, disposed rotatably in the main body such that a portion...http://www.google.ca/patents/US4968134?utm_source=gb-gplus-sharePatent US4968134 - Overhead projector
