LENGTH MEASURING INSTRUMENT

摘要

PURPOSE:To increase a stroke and to improve accuracy by providing one of two bodies which move relatively with a surface which slants in the relative moving direction and measuring the distance to said slanting surface by a position detecting means for a surface to be scanned which is arranged on the other body. CONSTITUTION:A diffraction grating GS is fixed to a moving stage SR on a surface plate SP in parallel to the moving direction A of the moving stage SR. Luminous flux emitted by a light source LD on a fine moving stage AFS is incident on the diffraction grating GS through a beam splitter HM2. Diffracted light beams L11 and L12 are reflected by corner cubes CC1 and CC2, diffracted again at a point P2 distant from the point P2 of the diffraction grating GS in the moving direction (arrow A) of the moving stage SR, and incident on photodetectors PD1 and PD2 through a polarization beam splitter BS. Then the quantity of the relative movement between a length measuring head MH and the diffraction grating is measured from the outputs of the photodetectors PD1 and PD2.