Low-loss birefringent spectral filters comprising three identical retarders.

Basic fan or folded arrangements comprising three identical retarders in cascade between parallel or crossed polarizers are presented for constructing birefringent spectral filters without an internal polarizer. The retarders' axes are oriented with an angle increment of theta or -theta preferably with /theta/ <or=34.27 degrees such that their orientation angles are, respectively, equal to 45 degrees -theta, 45 degrees , and 45 degrees +theta for the fan arrangement, or 45 degrees +theta, 45 degrees +2theta, and 45 degrees +theta for the folded arrangement. A filter in the fan or folded arrangement has the bandwidth of its principal transmission peaks or the amplitude of the sidelobes adjustable by changing the angle increment /theta/. For /theta/ = 25.91 degrees, the filter exactly produces the transmission spectrum of a standard Lyot two-stage filter. Further, the filter can have the spectral transmission be electrically tunable or switchable, depending on the retarders selected for use. The retarders can be birefringent plates, liquid-crystal variable retarders, or liquid-crystal electrically rotatable retarders. Test measurements and results are presented and discussed.