Patent ID: 8724114

Claim:
A method of calculating a geometric distance of a test optical path on the basis of interfering a test beam reflected by a test surface and a reference beam reflected by a reference surface, the method comprising: an optical-path-length calculating step of calculating an optical path length of the test optical paths by using the test beam and the reference beam each having a different wavelength; a refractive-index calculating step of calculating a refractive index of the test optical path based on the optical path length of the test optical path calculated by the optical-path-length calculating step; a smoothing step of smoothing a plurality of refractive indices acquired by repeating the optical-path-length calculating step and the refractive-index calculating step to calculate a smoothed refractive index; and a geometrical distance calculating step of calculating the geometrical distance of the test optical path based on the smoothed refractive index calculated by the smoothing step.