Patent ID: 7239447

Claim:
An objective, comprising: at least two lenses or lens parts of fluoride crystal, each lens or lens part having a lens axis that points approximately in a principal crystallographic direction, an image point in an image plane being impinged by a bundle of rays with rays which respectively have an azimuth angle α R , an aperture angle θ R and an optical path difference ΔOPL for two mutually orthogonal states of linear polarization, and each lens or lens part having optical surfaces; and a compensation coating on at least one optical surface, the compensation coating being designed in such a way that the distribution of the optical path differences ΔOPL (α R , θ R ) of the bundle of rays as a function of the azimuth angle α R and of the aperture angle θ R has significantly reduced values in comparison with an objective without a compensation coating, wherein the lenses or the lens parts are arranged such that they are rotated with respect to one another about the lens axes in such a way that the distribution of the optical path differences ΔOPL (α R , θ R ) of the bundle of rays as a function of the azimuth angle α R and of the aperture angle θ R has significantly reduced values in comparison with lenses or lens parts of which the lens axes point in the same principal crystallographic direction and which are not arranged such that they are rotated with respect to one another about the lens axes, and wherein an optical element with the compensation coating has an element axis, the compensation coating has an effective birefringence distribution, and the effective birefringence values of which depend on azimuth angles α F with respect to a reference direction perpendicular to the element axis and on aperture angles θ F with respect to the element axis.