This invention is a light valve using birefringent nematic liquid crystals in which compensation for residual birefringence is achieved by passing light through two separate liquid crystal layers having their major optical axes aligned perpendicular to one another at the interface between the two liquid...http://www.google.com.au/patents/US4466702?utm_source=gb-gplus-sharePatent US4466702 - Liquid crystal light valve with birefringence compensation
