Patent ID: 6958468

Claim:
A rotation detecting apparatus comprising: two members rotatable relative to each other, at least one of said two members forming on the other member at least two beams moving on concentric circles while forming a track as a circle or an arc in conformity with a relative rotation of said two members; a light receiving portion provided on said other member, said light receiving portion detecting incidence positions of said at least two beams; and an arithmetic system for calculating angle information respectively corresponding to at least two incidence positions selected from among a plurality of light incidence positions detected by said light receiving portion, calculating corrected angle information in which a decentered amount of a beam is corrected by adding the calculated angle information, calculating and outputting the angle between a straight line linking together at least two incidence positions detected by said light receiving portion and a predetermined reference line based on the calculation results, and detecting information concerning the relative rotation of said two members based on the output angle.