A spherical recess is provided to one end wall of a first planar lens and the opposite end walls of a second planar lens, the first planar lens having a thicknesswise index distribution in the vicinity of the one end wall whereas the second planar lens has a thicknesswise index distribution in the vicinity...http://www.google.se/patents/US5323268?utm_source=gb-gplus-sharePatent US5323268 - Compound lens
