As light passes through a optical system the reflections and refractions will in general change the polarization state of the light. If we assume that all of the materials in the thin film coating and substrate are isotropic and homogeneous then calculating the amount of "instrumental” polarization is a relatively straight forward task. In the following sections we will present all of the steps required to perform a "polarization ray trace" calculation including the thin film portion of the calculation. The reason for explicitly showing the thin film is that there are sign conventions imposed on the boundary value by the orientation and handedness of the various coordinate frames which are attached to the geometric rays.