A dual focal length ophthalmic lens is formed from a birefringent material with its fast and slow axes perpendicular to the user's visual axis. The dual focal property arises due to the differing indices of refraction of the birefringent material for light polarized parallel to the fast and slow axes....http://www.google.es/patents/US5073021?utm_source=gb-gplus-sharePatente US5073021 - Bifocal ophthalmic lens constructed from birefringent material
Bifocal ophthalmic lens constructed from birefringent material
