Patent ID: 8222594

Claim:
An encoder that detects positional information of a movable body, making use of interference of a first illumination light and a second illumination light, the encoder comprising: a scale that has a pattern arrayed in a movement direction of the movable body, and moves relative to the first illumination light and the second illumination light according to movement of the movable body: a light source that emits an illumination light: an optical system that generates the first illumination light and the second illumination light from the illumination light: an optical element that is arranged in an optical path between the optical system and a photodetection element, wherein the optical element respectively deflects the first illumination light and the second illumination light generated at the optical system and causes the first illumination light and the second illumination light to be incident on the scale: a modulator that includes an actuator, wherein the actuator periodically changes at least one of an optical path of the first illumination light and an optical path of the second illumination light by periodically oscillating the optical element based on a modulation signal, in order to causes a periodic optical path difference to occur between an optical path length of the first illumination light and an optical path length of the second illumination light; and a control circuit that generates the modulation signal and supplies the modulation signal to the modulator, wherein the optical element is made to periodically turn around an axis parallel to a direction orthogonal to the movement direction by the actuator periodically oscillating the optical element based on the modulation signal.