An achromatic half wave plate includes a first twisted nematic liquid crystal layer, a second twisted nematic liquid crystal layer, and a uniaxial half wave plate between the first twisted nematic liquid crystal layer and the second twisted nematic liquid crystal layer. In one embodiment the first twisted...http://www.google.com.au/patents/US6765635?utm_source=gb-gplus-sharePatent US6765635 - Apparatus and method for achromatic liquid crystal electro-optic modulation
