A marker apparatus including a retroreflector constituted by three reflective surfaces arranged on three surfaces of a trigonal pyramid and a light source disposed on an optical axis of the retroreflector. A lens is provided in front of the retroreflector and a condensing lens is formed on the lens for...http://www.google.ca/patents/US5580156?utm_source=gb-gplus-sharePatent US5580156 - Marker apparatus
