Optical sensor for detecting a code on a substrate which, in use, moves with respect to the optical sensor. The sensor comprises an optical detector for determining a value of a reflective property of the substrate, wherein the optical detector views the substrate along an optical axis, a code analyser for determining the detected code on the basis of a variation of the value of the determined reflective property in time, a first illumination source for illuminating the substrate along the optical axis, a second illumination source for illuminating the substrate in a direction at a non-zero angle with respect to the optical axis, and a control unit having a first operational mode wherein the first illumination source switched on and the second illumination source is switched off, and having a second operational mode wherein the first illumination source is switched off and the second illumination source is switched on.