Abstract: A filter element for a narrow-passband optical filter comprises a birefringent crystal (10) having a rectangular parallelopiped configuration with an entrance face (11), an exit face (12) and side walls (21, 22, 23, 24). The optic axis (25) of the crystal (10) is parallel to the entrance and exit faces (11 and 12). The side walls (21, 22, 23, 24) are polished to provide total internal reflection of optical energy incident thereon at greater than a critical angle as measured from the normal. By covering the side walls (21, 22, 23, 24) with a coating whose index of refraction is given by the algorithm n=(N.sup.2 =sin.sup.2 .theta.).sup.1/2, where n is the index of refraction of the coating, N is the lower index of refraction of the crystal (10), and .theta. is a half-angle defining the maximum field of view in air for the filter element, the critical angle for total internal reflection is made substantially equal to the half-angle defining the maximum field of view.

Abstract: A birefringent filter module comprises, in seriatum, an entrance polarizer, a first birefringent crystal responsive to optical energy exiting the entrance polarizer, a partial polarizer responsive to optical energy exiting the first polarizer, a second birefringent crystal responsive to optical energy exiting the partial polarizer, and an exit polarizer. The first and second birefringent crystals have fast axes disposed .+-. 45.degree. from the high transmitivity direction of the partial polarizer. Preferably, the second crystal has a length 1/2 that of the first crystal and the high transmitivity direction of the partial polarizer is nine times as great as the low transmitivity direction. To provide tuning, the polarizations of the energy entering the first crystal and leaving the second crystal are varied by either rotating the entrance and exit polarizers, or by sandwiching the entrance and exit polarizers between pairs of half wave plates that are rotated relative to the polarizers.