An optical readhead for measuring a displacement along a measuring axis direction between the readhead and a scale track comprises an illumination portion configured to output source light to the scale track and a monolithic detector configuration. The monolithic detector configuration comprises: a first track photodetector portion configured to receive scale light from the scale track; a first metal layer; a second metal layer; and a plurality of dummy vias between the first metal layer and the second metal layer. The plurality of dummy vias is arranged to block light transmission along a layer between the first and second metal layers, and the plurality of dummy vias is formed by the same process steps used to fabricate a plurality of active vias used to connect circuit elements on the monolithic detector configuration.