Four groups of six lenses comprising a plastic lens having weak refracting power, three glass lenses of convex, concave, and convex yielding a major part a refracting power of the projection lens as a whole, a plastic lens having a weak refracting power, and a lens having a concave surface directed onto...http://www.google.fr/patents/US6061185?utm_source=gb-gplus-shareBrevet US6061185 - Projection lens having all glass lenses in a second lens group
Projection lens having all glass lenses in a second lens group
