Reflection ellipsometry and computational modeling are used to examine electro-optic (EO) effects in a PZT (30/70) thin film. The combined influences of thin film etalon effects, the optical configuration used to measure electro-optic changes in the PZT/electrode thin film stack, and incoherent scattering within the layer structure are found to significantly influence the corresponding intensity modulation under an ac field. The impact of these findings on design considerations for EO-based devices and the use of photometric ellipsometry to characterize these materials is discussed.