A bifocal contact lens has diffractive power added to the basic refractive power provided by the material of the lens and the basic curvature of its front and rear surfaces. The diffractive power arises from concentric zones, each providing an asymmetric retardation of light across the zone width to...http://www.google.com/patents/US4637697?utm_source=gb-gplus-sharePatent US4637697 - Multifocal contact lenses utilizing diffraction and refraction
