A sensor for sensing contamination in an application system is disclosed. In one aspect, the sensor comprises a capping layer. The sensor is adapted to cause a first reflectivity change upon initial formation of a first contamination layer on the capping layer when the sensor is provided in the system. The first reflectivity change is larger than an average reflectivity change upon formation of a thicker contamination layer on the capping layer and larger than an average reflectivity change upon formation of an equal contamination on the actual mirrors of the optics of the system.