| 摘要 |
PROBLEM TO BE SOLVED: To prevent degradation in length-measurement precision due to changes in the environmental temperature. SOLUTION: There are provided a light source, a diffraction lattice which splits optical flux into two optical fluxes, transmission light and diffraction light, a spindle movable together with the diffraction lattice integrally in the direction vertical to the line direction of diffraction lattice a liquid crystal device, which is provided on one optical path of the two optical fluxes, phase- modulated, so that a desired phase difference is provided in the regions in a cross-section plane vertical to the advancing direction of the optical flux, a reflective mirror which makes the two optical fluxes to be incident again on an incidence point of the diffraction lattice, and a photoelectric conversion member, which converts the interference light from two optical fluxes re-combined at the diffraction lattice into a plurality of electrical signals, corresponding to the phase modulation region of the liquid-crystal device. Here, the displacement of diffraction lattice is measured, based on a plurality of output signals from the photoelectric conversion member. The diffraction lattice is divided into two members at a position vertical to the line direction, which are held with spindles of different materials with different linear expansion coefficients.
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