This invention relates to a focusing method for interferometer.
Various types of interferometer have been known. One of them is an interferometer in which a testing beam from a tested surface and a reference beam are caused to interfere with each other, and another interferometer is of a shearing type in which a light beam from a tested surface is split into two sub-beams to thereby split the wave front of the testing beam into two segments which are displaced transversely and made to interfere with each other. When the accuracy of the surface of a mirror or lens is tested for irregularities by using an interferometer, for example, it is necessary to perform focusing to place the tested surface in a position in which testing can be carried out. To this end, it has hitherto been usual practice to perform a focusing operation in such a manner that the tested surface is first placed in a position which is assumed to be the focusing position in the direction of its center axis to produce an interference fringe.
Meanwhile, to ascertain the contour of the tested surface by testing same, it is necessary to learn the radius of curvature of a wave front (reference wave front) applied to the tested surface at the time testing was performed. Since the test is based on the determination of a difference (or a value corresponding to the difference) in configuration between the reference wave front and the actual tested surface, it would be impossible to accurately determine the contour of the actual tested surface unless the radius of curvature of reference wave front is known. When the method of the prior art referred to hereinabove for effecting focusing is used, difficulties are experienced in accurately determining the radius of curvature of the reference wave front because the method merely brings the tested surface to an optimum position (focusing position).