Patent ID: 7544925

Claim:
An encoder comprising: a rotating member; a light source device for emitting light to be detected to the rotating member; and a photodetecting device including a scale having a plurality of photodetecting elements arranged thereon, and an output portion for outputting an output signal based on the light intensity of the light to be detected made incident to the photodetecting elements through the rotating member, wherein the photodetecting elements are arranged along an annular arrangement line on the scale, and a semicircular light transmission portion whose diameter corresponds to the arrangement line is formed in the rotating member, and the rotating member is designed so that, in the scale, an area containing a part of the arrangement line is set as a bright portion to which the light to be detected passing through the light transmission portion is irradiated, and an area containing the other portion excluding the part of the arrangement line concerned is set as a dark portion to which no light to be detected is irradiated, and the encoder further comprises: a detector for detecting an angle associated with a reference point on the basis of angle information allocated to a photodetecting element corresponding to the center at half maximum of a light intensity waveform obtained from a one-dimensional profile acquired on the basis of output signals from the plurality of photodetecting elements, and for specifying the photodetecting element corresponding to the reference point.