A focusing plate comprising two base plates, a liquid crystal layer held between the two base plates, and an electrode pattern which is formed on each of the two base plates for the purpose of rotating the liquid crystal molecules in the liquid crystal layer by applying an electrical field to the liquid...http://www.google.de/patents/US5122888?utm_source=gb-gplus-sharePatent US5122888 - Focusing plate having phase grating formed by using liquid crystal
