Optical linear encoder with light quantity calabration

Claim 1

An averaged diffraction moire position detector which is capable of detecting the relative displacement of a diffraction grating in the lateral direction by using a signal change such that half the pitch P of a diffraction grating which appears at an average value is used as a cycle, comprising: a first diffraction grating; a second diffraction grating which is displaced in the lateral direction with respect to the first diffraction grating; optical-path length adjustment means, disposed between said two diffraction gratings, for varying the optical-path length of the gap between said two diffraction gratings over the range of an optical-path length equivalent to a Fresnel number 2 or an integral multiple of 2 for each part of an effective area of said two diffraction gratings; and average value obtaining means for obtaining a signal equivalent to the average value of the diffraction moire signals over the effective areas of said two diffraction gratings, wherein said averaged diffraction moire position detector includes a calibration means for calibrating a relative displacement signal of said diffraction grating in the lateral direction obtained by detecting even-number-order diffracted lights among respective order diffracted lights diffracted by said first and second diffraction gratings, by a calibration signal obtained by detecting odd-number-order diffracted lights among said respective order diffracted lights.