| 摘要 |
PROBLEM TO BE SOLVED: To permit the measurement of natural decentration in an off-axis state by correlating wavefront errors in different regions received by at least one of the first and second test surfaces before and after rotation through the use of a dummy primary standard in the state of base material in the case that a specimem is what is called off-axis to its outer diameter. SOLUTION: A different point from before is that a dummy primary standard 7 in the state of base material is used, and the reason that the primary standard 7 is used is that difference data before and after rotation can not obtained as there is no overlapping regions before and after rotation. Interference measurement is performed in advance on the overall surface of a wavefront to be measured through the use of a reflector primary standard, and the obtained data is stored as reflection data in the computing unit of an interferometer. The reflection data is subtracted at the time of the interference measurement of test surfaces 5a and 5b. In decentration measurement, spherical surface measurement is performed by a backside Fizeau interferometer on the backside at two positions of 0 deg. and 180 deg. to compute natural decentration to the backside through the use of two items of calibrated data of the side of an aspheric surface at the same rotational position with respect to a Fizeau surface 3a. In addition, calibration may be performed through the use of a spherical-surface primary standard in the state of base material.
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