US Patent 7723671 Positional information detecting device

A light via first and second index scales is split by a beam splitter, and one of the split lights is received by a first light-receiving element via a movable scale and also the other of the split lights is received by a second light-receiving element via a reference scale, and therefore by computing positional information of the movable scale using an output of the first light-receiving element (a first output) and an output of the second light-receiving element (a second output), movement information of the movable scale can be measured with high precision without being affected by drift of the modulation center (the oscillation center) of the beam.