Patent ID: 8339611

Claim:
A measurement apparatus configured to measure a distance between a reference surface fixed on a fiducial surface and a test surface located on a test object, the apparatus comprising: an optical frequency comb generation unit configured to generate a light beam with a plurality of optical frequency components, which have equal optical frequency separations therebetween; a detection unit configured, for at least two of the plurality of optical frequency components, to detect an interference signal between a portion of the light beam reflected by the reference surface and a portion of the light beam reflected by the test surface to detect a phase corresponding to an optical path length between the reference surface and the test surface; and a calculation unit configured to calculate a geometric distance between the reference surface and the test surface based on the phases detected by said detection unit, wherein for each of the plurality of optical frequency components, said calculation unit is configured to calculate the geometric distance by converting the phase detected by said detection unit into an optical path length, and fitting a function to the optical path length, and wherein the function is expressed by the geometric distance and amount of change in optical length obtained by multiplying the refractive index of a medium, which is present between the reference surface and the test surface and has a known dispersion characteristic, by the geometric distance.