The optical mode structures of solvent cast films of different polymers have been measured with a single prism film coupler. The allowed modes define the index of refraction for light polarized in and perpendicular to the plane of the film. It is shown that the solvent coating process preferentially aligns the polymeric chain backbone in the plane of the film and that this biaxial orientation may produce birefringent films in optically anisotropic resins. The sign of this birefringence depends upon the orientation of the dominating polarizable group relative to the chain backbone in the plasticized melt. The magnitude of this effect reflects both the optical anisotropy of the group and the steric constraints to its free rotation.