Patent ID: 8345263

Claim:
A measurement method of illuminating a surface to be tested having an aspheric surface using light beams that form spherical waves, and measuring a figure of the surface to be tested, comprising: a first detection step of positioning the surface to be tested at multiple positions by moving the surface to be tested along a direction of an aspheric surface axis of the aspheric surface, with a center of curvature of the spherical waves aligned with the aspheric surface axis, and detecting interference patterns between light beams from the surface to be tested and light beams from a reference surface at each of the multiple positions, a second detection step of positioning the surface to be tested at multiple shift positions at which the surface to be tested has been shifted from the multiple positions respectively by a known amount in the direction of the aspheric surface axis, and detecting interference patterns between light beams from the surface to be tested and light beams from the reference surface at each of the multiple shift positions, a first calculation step of calculating a length between a center of curvature of the spherical waves and a measurement position on the surface to be tested at which the light beams are perpendicularly incident based on the interference patterns detected in the first detection step and the second detection step for each of the multiple positions and each of the multiple shift positions, a second calculation step of calculating, based on a calculation result calculated in the first calculation step, change in the length when positioning of the surface to be tested was performed at the multiple positions relative to when the positioning was performed at the multiple shift positions, and a third calculation step of determining the measurement position for each of the multiple positions based on the change in the lengths calculated in the second calculation step, and calculating a figure error, which is a difference between the length obtained from a design value at the determined measurement position of the surface to be tested and the length calculated in the first calculation step for each of the multiple positions.