Patent ID: 7848000

Claim:
An optical wavelength filter comprising: a plurality of birefringent retarders and at least one polarizer disposed along a light propagation path defining an optical axis, wherein the retarders comprise stacked plates normal to the axis, rotationally oriented relative to one another and relative to the polarizer so as to discriminate for at least one of a passband and a stopband at a discrimination wavelength; wherein the filter is configured to filter the discrimination wavelength over a filter field of view through the stacked plates that is at least ±7.5° oblique to the axis; wherein the retarders comprise paired fixed birefringent plates and liquid crystals, abutted with one another such that at least one of said fixed birefringent plates and at least one of said liquid crystals have fast and slow axes that are aligned, and further comprising a controller operable to tune at least one stage containing said liquid crystals for adjusting the discrimination wavelength; and wherein the plates are configured in at least two cascaded stages and wherein at least one of the stages has paired birefringent plates and liquid crystals selected from the set consisting of: two retarders respectively oriented at 22.5° and 67.5° relative to a reference angle; three retarders respectively oriented at 12.5°, 45° and 77.5° relative to a reference angle; and, four retarders respectively oriented at 7.5°, 29.5°, 60.5° and 82.5° relative to a reference angle; wherein the reference angle is one of parallel to and orthogonal to the polarizer.