Bifocal contact lenses are lenses with two zones having different optical powers, or more than two zones in which case they are also called multifocal lenses. Concentric bifocal contact lenses have two or more annular power zones with a connection that presents optical problems. These optical problems...http://www.google.co.uk/patents/US6176579?utm_source=gb-gplus-sharePatent US6176579 - Bifocal contact lens with toric transition
