An illumination portion of an optical encoder comprising a scale track extending along a measuring axis direction, an imaging portion, and a detector configuration. The illumination portion comprises: a light source configured to output source light; a collimation portion; and a structured illumination generating portion comprising a beam-separating portion and an illumination grating and configured to input the source light and output structured illumination to the scale track. The beam-separating portion is arranged to input the source light and output a first source light portion and a second source light portion to the illumination grating, such that they form beams that are spaced apart from one another along the measuring axis direction. The illumination grating is configured to diffract the first and second source light portions to the scale track such that only two orders of diffracted light overlap within an imaged region.