Patent ID: 8445835

Claim:
An encoder, comprising: a circular disk that is disposed rotatably about a rotary axis and that has a first track and a second track each being ring-shaped and provided with an optical rotary grating; and a first detector and a second detector that are fixedly disposed so as to respectively face the first track and the second track each provided with an optical fixed grating that structures a diffraction interference optical system in conjunction with the facing rotary grating of corresponding one of the first track and the second track to detect diffracted interfering light obtained by the rotary grating and the fixed grating, wherein the rotary grating of the first track includes a plurality of slits formed as curved slits each curved from a radial direction, and the first detector facing the first track is disposed so as to face the first track at a position where a tangent of each of the slits included in the rotary grating of the first track becomes parallel to a tangent of each of the slits included in the rotary grating of the second track at a position where the second detector faces the second track.