This paper establishes that any thin periodic metal grid on a plane boundary between two semi-infinite media, irradiated normally at any specified wavelength larger than its period, can be represented by an equivalent thin film. Explicit formulas for the parameters of this film are derived, and a numerical example of the equivalence is given.
Philip E. Ciddor and Lewis B. Whitbourn, "Equivalent thin film of a periodic metal grid," Appl. Opt. 28, 1228-1230 (1989)