A bifocal lens of the Cohen lens design utilizing phase zone plate optics and a facet depth of one-half wavelength of the design wavelength, where the primary focal points are at two orders, specifically the 0.sup.th and 1.sup.st orders, and the brightness at each primary focal point is equal at about...http://www.google.es/patents/US5144483?utm_source=gb-gplus-sharePatente US5144483 - Diffractive multifocal optical device
