An illumination portion for an optical encoder is disclosed. The optical encoder comprises the illumination portion, a scale grating extending along a measuring axis direction and arranged to receive light from the illumination portion, and a detector configuration arranged to receive light from the scale grating. The illumination portion comprises an addressable light source array. The addressable light source array comprises individual sources arranged along the measuring axis direction. The addressable light source array is configured to provide at least a first addressable set and a second addressable set of the individual sources.