A rotary angle sensor for an electric power assisted steering system of a motor vehicle, wherein the rotary angle sensor comprises an optical sensor unit with a light source, optical components and a photodetector and a disc with an optical pattern, wherein the optical sensor unit and the disc are configured to rotate relative to each other around a rotary axis, and wherein the optical sensor unit is configured such that light reflected from the optical pattern is measured by the photodetector, wherein the optical pattern comprises steps and spaces separating the steps, such that light reflected by steps and spaces destructively interferes leading to an intensity modulation of the reflected light according to an optical pattern which encodes a binary type code for the rotary angle of the disc.