A lens system has a lens unit comprising a positive lens and a negative lens. The positive lens has an index distribution in the direction of the optic axis. The value of the index gradient for a light having a short wavelength in the vicinity of the vertex of at least one convex surface of the positive...http://www.google.se/patents/US4810070?utm_source=gb-gplus-sharePatent US4810070 - Lens system having a good image performance for white light
