Abstract: An optical system, for example an illumination system or a projection objective (10), of a microlithographic projection exposure apparatus contains an optical element (L2, L3) which consists of a birefringent material. A projection light beam (14) formed by linearly polarized light rays passes through the optical element (L2, L3). In order to avoid perturbations of the polarization distribution of the light beam, the birefringent material is aligned such that each light ray entering the material is polarized substantially parallel or substantially perpendicularly to a slow birefringent axis for the respective light ray.