Patent ID: 8658964

Claim:
A photoelectric encoder comprising: a light source configured to emit light; a scale having an optical grating that is disposed at constant intervals and being configured to modulate the light emitted from the light source by the optical grating; a light receiving element configured to detect the light modulated by the optical grating; and an imaging optical system configured to form an image of the light modulated by the optical grating, wherein the imaging optical system has a first aperture through which part of the light modulated by the optical grating is allowed to transmit, the first aperture having a size based on a cutoff spatial frequency, when the cutoff spatial frequency has a value between a fundamental spatial frequency of the optical grating obtained by a Fourier transformation and a second harmonic of the fundamental spatial frequency, the cutoff spatial frequency being a spatial frequency indicating a resolution limit of the imaging optical system due to the light emitted from the light source, and wherein the scale is inclined by an error angle δθ with respect to an optical axis O of the encoder due to a placement error of the scale, so that an upper flux angle θ 4UP at an upper side with respect to the incident main light beam originating from the light source, a light source flux angle θ 4 of the incident main light beam originating from the light source, a detection light flux angle φ 4 of an emission main light beam and the error angle δθ satisfy the expression: ⊖ 4UP =θ 4 −2×δθ>φ 4 .