Plate-void model for thin-film form birefringence

A plate-void model for thin-film form birefringence is shown to be equivalent optically to an array of elliptical columns. With voids distributed according to f(θ) = a + b sin θ, the principal refractive indices are given by the Bragg–Pippard equations with depolarization factors L1 = 0, L2 = 1/2 + b/3(1 − p), and L3 = 1/2 − b/3(1 − p), where p is the packing density. The possible use of form birefringence in thin-film devices is considered briefly.