The waveguide mode equation is considered for uniaxial and biaxial film microstructures, where one of the principal axes os symmetry is assumed to be orthogonal to the plane of incident. PbF2 films thermally deposited at normal incidence show an amount to birefringence ?n which is negligible compared to the mean deviation error ? of the experiment. However the isotropic assumption becomes unsustainable for CeO2 films deposited at a vapor incidence angle of 58, 3°, leading to ratios (?n/?) ? 20. Under such conditions, a biaxial columnar model for the film microstructure provides good consistency in the interpretation of the measurement results. Combined with those from the envelop method, the model allows determination of the principal refractive indices as n1=1,65 ± 0,05, n2= 1,794 ± 0,002 and n3= 1,97 ± 0,06.
