A light mixing system for use in a photographic enlarger in which light from a light source is admitted to a light mixing module through an aperture (and/or through a light modifying filter system for color correction for color enlargements) to mix light beams to provide substantially uniform energy...http://www.google.com.au/patents/US4240738?utm_source=gb-gplus-sharePatent US4240738 - Light mixing system for a photographic enlarger
