Development of an aspherical shape measurement system for on-machine

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To measure an aspherical surface processed by an ultra precision lath, an on-machine measurement system was developed as follows:1)Shearing interferometer : A shearing interferometer was developed to measure the asperical surface on the ultra precision lath just after the processing. Because a part of an interferogram was covered with a tool post, the intensity of the interferogram acquired by a line CCD camera was analyzed. And a zone plate was used to generate the aspherical wave front.2)Software : Software was developed to analyze the interferograms. The interferograms were obtained every time that the measured surface was turned fixed angle. Therefore, each interferogram was analyzed to obtained a line shape passing through the turning center and the line shapes were assembled to construct aspherical surface. And an algorism was developed to fit a curve into measured shape.3)Experiment and results : A spherical mirror with shape error in center point was measured to estimate an ability of developed system. The result obtained by developed system agree well with the result obtained by Fizeau interferometer.4)Proposal of new principle : A method of measurement using many sphere wave fronts with various curvatures was proposed to measure these aspherical surfaces. Validity of proposed method was confirmed by simulations and experiments.