Narrowband birefringent filters have traditionally been used by solar physicists to simultaneously isolate narrow spectral bands over wide spatial regions of the sun. Recent development of optical communication and detection systems, however, has stimulated broader interest in birefringent filters because of their unique filtering abilities. Tenability and large fields of view make these filters attractive for imaging and non-imaging applications. The purpose of this paper is to illustrate the capabilities of birefringent filter concepts by presenting examples of actual working units which have been built at the Lockheed Solar Observatory. An adequate theoretical background is available in the literaturel'2'3 but practical considerations are generally neglected. As a result, there is little appreciation for the capabilities and limitations of current birefringent filter technology. The following discussion will concentrate on the practical aspects of the filters and will also consider the restrictions imposed by material limitations. Although this presentation is mainly concerned with the practical capabilities of the filters, brief discussions of the theoretical foundations are included for convenience. Specifically, short descriptions of the basic polarization interference principle, the wide field element construction and the tuning technique are included.
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