Patent ID: 8258459

Claim:
An optical encoder comprising: a rotation disc attached to a rotation axis and rotatable together with the rotation axis; an annular slit provided to said rotation disc and including multiple equal-pitched concentric slit patterns formed to be decentered as to a rotation center of said rotation disc, said annular slit including a first annular slit and a second annular slit which are decentered in a mutually different direction; a light source configured to irradiate light on said annular slit; an absolute-value-related detecting unit configured to detect transmitted light or reflected light irradiated from said light source and transmitted or reflected at said annular slit to output a detected signal, said absolute-value-related detecting unit including a first detecting unit corresponding to the first annular slit and a second detecting unit corresponding to the second annular slit; and a signal processing device configured to detect an absolute rotational angle of said rotation disc based on the detected signal output from said absolute-value-related detecting unit.