Polar curves for transmission ellipsometry

For three-phase ambient-film-substrate systems, the ratio ?_T = T_p/T_s of the complex amplitude-transmission coefficients T_p and T_s for light polarized parallel (p) and perpendicular (s) to the plane of incidence is related to a complex exponential function of film thickness d by a bilinear transformation. This observation leads to elucidation of the nature of the complex plane polar curves of the function ?_T(d), where the film thickness is variable and the angle of incidence is fixed. In particular, the polar curve ?_T(d) for a transparent ambient-film-substrate system studied by transmission ellipsometry is shown to be exactly a circle.