An apparatus comprising at least one light source, at least one photodetector, a first layer of optical material configured to embed the at least one light source, and a second layer of optical material configured to embed the at least one photodetector. The first and second layer of optical material are configured to guide light from the at least one light source and to prevent the light from the at least one light source directly reaching the at least on photodetector, and the second layer of optical material is configured to guide light towards the at least one photodetector.