Patent ID: 7649168

Claim:
An optical encoder comprising: a light source; a photodetector; a scale which is relatively displaceable with respect to the light source and the photodetector; a first grating which is interposed between the light source and the scale, and on which, an optical pattern of a first pitch p 1 is formed in a direction of relative displacement of the scale; a second grating which is provided on the scale, and on which, an optical pattern of a second pitch p 2 is formed in the direction of relative displacement of the scale; and a third grating which is interposed between the scale and the photodetector, and on which, an optical pattern of a third pitch p 3 is formed in the direction of relative displacement of the scale, wherein light emerged from the light source travels via the first grating and the second grating, and forms an image on the third grating as a magnified scale-image pattern in which the optical pattern formed on the second grating is magnified at a predetermined magnification, and a periodic signal according to an amount of relative displacement of the scale is detected, and the first grating and the third grating are formed on separate members, and values of a first effective width W 1 and the first pitch p 1 of the optical pattern on the first grating, and a third effective width W 3 and the third pitch p 3 of the optical pattern on the third grating are set to values such that the periodic signal having amplitude effective for detection of the relative displacement of the scale is achieved based on periodicity of the magnified scale-image pattern, refractive indices of substances and/or spaces interposed in the optical path from the light source up to the photodetector, and the thickness of those substances and/or spaces in a direction substantially perpendicular to the plane on which the second grating is formed.