DEVICE FOR PHOTOELECTRIC LENGTH MEASUREMENT OR ANGLE MEASUREMENT

PROBLEM TO BE SOLVED: To obtain a photoelectric length or angle measuring apparatus in which the measurements are insensitive to the local fluctuation in the measuring graduations being scanned and relatively insensitive to the fluctuation of scanning interval by deflecting at least a part of partial beam flux passed through a partial scanning measurement field of view in the direction of common partial region of a dial. SOLUTION: A light passes, at first, through a scanning plate 35 and impinges on a dial 45 where it is reflected and diffracted to pass through the scanning plate 35. After passing through the scanning plate 35 twice, the luminous flux arrives at a detection element which detects a modulation signal dependent on the position in case of relative movement. Deflection means 3d5, 3e5, 3f5 performing direction selectivity action on the rear side 37 of a scanning plate 35 deflects the light to a common region 4b5 of measuring graduations 4a5 being scanned by the entire incident luminous flux from partial scanning measuring fields of view 3a5, 3b5, 3c5 producing phase shifted signal components thus guaranteeing the single field scanning. The phase shifted signal components are separated spatially by diffraction at the second time passage of luminous flux through the scanning plate 35 and detected on the focal plane of a condenser optical system.