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.