LENGTH MEASURING INSTRUMENT

摘要

PURPOSE:To improve the reliability of a light source to wavelength variation and to improve the measurement accuracy by providing two gratings, i.e. fixed grating and moving grating, and refracting the (+ or -1)th-order light of the fixed grating by the moving grating again and guiding the diffracted light to a photosensor. CONSTITUTION:Luminous flux which is emitted by the light source LD and made into parallel light by a collimator lens CL is split into two by a beam splitter HM1; and one is incident on an objective LN and the other is incident on a diffraction grating GS through a beam splitter HM2. The (+ or -1)th-order light beams which are diffracted by the diffraction grating GS are incident on the diffraction grating GS again through phase difference plates EP1 and EP2 and corner cubes CC1 and CC2. The light which is diffracted by the diffraction grating GS again is incident on photodetectors PD1 and PD2 through the polarization beam splitter BS. Output pulses of the photodetectors PD1 and PD2 are counted to measure the relative movement quantity between a length measurement head MH and the diffraction grating GS.