Transmissive quarterwave retardation plates have been successfully fabricated recently using birefringent thin films (~3 μm thick) obtained by oblique-incidence deposition of metal oxides on glass slides.1 The desired retardance is acquired in one-way transmission through the film at normal incidence and the effect of (incoherent) multiple-beam interference was apparently negligible. In this paper light interference in a birefringent thin film deposited on a reflecting (typically metallic) substrate for normally incident coherent illumination is considered. The object is to design birefringent mirrors that function as quarterwave and halfwave retarders in normal-incidence reflection. For a given substrate at a given wavelength, the constraint on the birefringence, average refractive index, and thickness of the film so that quarterwave or halfwave retardation is attained at normal incidence is determined. A 2-D Newton-Raphson method is utilized to find all possible solutions. The sensitivity of a given design (i.e., the deviation from exact quarterwave or halfwave retardation) is calculated for given perturbations of the film parameters.