Patent ID: 7692746

Claim:
An optical filter for filtering a beam of light passing through it, the optical filter comprising: a retarder stack, the retarder stack comprising at least three retarder layers wherein the retarder layers have substantially the same retardance values, and the retarder angles of the retarder layers are substantially symmetric relative to the middle of the retarder stack; a liquid crystal cell adjacent to the retarder stack, the liquid crystal having a voltage input and operable at least at a certain input voltage signal at its voltage input to impart no change of the state of polarization of the light beam passing through the optical filter; and first and second polarizers on opposite sides of the adjacent retarder stack and liquid crystal cell such that polarization of the beam of light remains substantially unchanged between the liquid crystal cell and the one of the first and second polarizers that is adjacent to the liquid crystal cell.