Light wave interference length-measuring apparatus

Claim 1

A light wave interference displacement measurement apparatus which uses two beams of different wavelengths of coherent light comprising: a light source (7) for causing two coherent light beams having a first wavelength λ a and a second wavelength λb, respectively, incident upon a common light path for measuring at least the length of said light path; an optical element (6) for splitting at least one beam from said light source into a reference light path and a light path of length to be measured, and for combining the same; a movable light reflector means (1) arranged in said light path of length to be measured to reflect the incident light on a substantial incident light path; separation means (10, 6'), arranged in the vicinity of said optical element for splitting and combining, to separate reflected light from said movable light reflector means into separate beams of said first and second wavelengths, respectively; a first light detector means (4) for detecting a phase difference δa resulting from the combination of the reflected beam of the first wavelength from the movable light reflector means and the beam from the reference light path; a second light detector means (4) for detecting a phase difference δb resulting from the combination of the reflected beam of the second wavelength from the movable light reflector means and a reference beam; and signal processing means (5) for calculating, from the phase differences δa and δb detected by said first and second light detector means, an error resulting from an apparent moving distance of said movable light reflector means and a flicker of the atmosphere in order to detect the true displacement of said movable light reflector means.