It is shown that the performance of metal-Perot and Sp acerless Induced Transmission types in parallel radiation incident obliquely can be described in terms of that of a single equivalent dielectric layer of refractive index µE. Expressions for µE are deduced in terms of the optical constants of the layer materials, and their validity is established by the satisfactory agreement that is shown to exist between experimental and calculated values of µE.
D. J. Hemingway and P. H. Lissberger, "Effective Refractive Indices of Metal—Dielectric Interference Filters," Appl. Opt. 6, 471-476 (1967)