Optical interferometer

An optical interferometer that calculates a geometrical distance of an optical path length difference between a reference surface and a target surface by measuring the optical path length difference using a multi-wavelength light source for interference measurement, the optical interferometer comprising:

• a light source for partial pressure measurement;
• a dispersion measurement interferometer for measuring a dispersion of air for wavelengths of the multi-wavelength light source;
• a first partial pressure detector for measuring a partial pressure of a component gas in air in the dispersion measurement interferometer;
• a distance measurement interferometer for measuring the optical path length difference between the reference surface and the target surface with respect to the wavelengths of the multi-wavelength light source;
• a second partial pressure detector for measuring a partial pressure of the component gas in air in the distance measurement interferometer;
• and
• an analyzer configured to calculate an air dispersion ratio of air in the dispersion measurement interferometer excluding the component gas on the basis of a detection result of the first partial detector, to calculate dispersion of the air in the distance measurement interferometer from the calculated air dispersion ratio and a detection result of the second partial pressure detector, and to calculate the geometrical distance of the optical path length difference between the reference surface and the target surface.