A figure of merit for iso-index filter materials

A discussion of materials for dispersive birefringent filters which simultaneously exhibit extremely narrow bandwidths (< 1 Å) and wide fields of view (
\geq +45\deg
acceptance angle) will be presented in this paper. Two specific types, the iso-index filter and the dispersive birefringent Lyot filter are considered. First, a figure of merit for the filter material is derived. Then the figure is related to the optical properties of the material. From this analysis the dependence of the filter bandwidth on crystal birefringence and optical bandedge characteristics is obtained.