In reflectivity measurements on submicron-embossed polystyrene films, we found that form-birefringent polymer gratings can be described by generalized Fresnel equations for a biaxially anisotropic film in an optically isotropic medium. Thus, polarizedlight emitting illumination systems may be envisioned that basically consist of an optically isotropic substrate, into which light is coupled from an external light source, and a second layer, which comprises a form-birefringent, submicron relief structure on the bottom surface that acts as polarization-selective mirror and a light out-coupling structure on the top surface. Such a design should simplify device manufacturing considerably as the main parts of the systems could be produced by injection molding using inexpensive bulk polymers.