Patent ID: 7436520

Claim:
A method for calibrating an interferometer apparatus for measuring an optical surface, the interferometer apparatus comprising a light source, a camera having a plurality of detector elements, a reference surface and an interferometer optics, wherein the interferometer optics is configured to supply a beam of light generated by the light source to the reference surface and to the optical surface to be measured when the optical surface is positioned at a measuring position of the interferometer apparatus, wherein the interferometer optics is configured to image the reference surface and the optical surface onto the camera such that an interference pattern is generated by wavefronts that are reflected from the reference surface and that interfere with wavefronts interacting with the optical surface, the method comprising: positioning a calibrating element at a calibrating position of the interferometer apparatus; forming a first interference pattern on at least a portion of a light sensitive substrate of the camera, wherein the first interference pattern is generated by wavefronts that are reflected from the reference surface and that interfere with wavefronts interacting with the calibrating element, wherein the calibrating element is configured such that the first interference pattern is an interference pattern of a predetermined type or a superposition of the interference pattern of the predetermined type and a second interference pattern, and wherein the calibrating element is configured such that the interference pattern of the predetermined type comprises interference fringes arranged in two dimensions according to a predetermined two-dimensional pattern; and determining phase shifts of the first interference pattern in dependence upon detector signals generated by the detector elements located in the portion of the light sensitive substrate where the first interference pattern is formed, and determining imaging properties of the interferometer optics in dependence upon the determined phase shifts wherein the interference fringes of the interference pattern of the predetermined type are arranged in two dimensions according to a predetermined two-dimensional pattern such that predetermined two-dimensional pattern has periodicity in both a first direction and a second direction different from the first direction.