Patent ID: 8637805

Claim:
An optical encoder comprising a scale, which has a scale track extending along a reading direction and comprising scale markings, and a readhead, which includes a light source that emits light to the scale track and a light receiving element that is arranged to receive the light reflected by the scale track, so that a position of the readhead with respect to the scale is measured based on the light received by the light receiving element, wherein: the readhead comprises a first lens that transmits the light emitted from the light source to the scale track and transmits the light reflected by the scale track as scale track image light to the light receiving element; the light source is arranged on a light source plane located between a plane of the first lens and a plane of the light receiving element; and a center of the light source is separated from an optical axis of the first lens by an illumination off-axis distance Dsrc in a direction perpendicular to the reading direction of the scale track.