The invention provides a level sensor to measure a position of a surface of a substrate, comprising a projection unit arranged to direct a beam of radiation to the surface of the substrate and a detection unit. The detection unit comprises a detection grating arranged to receive the beam of radiation reflected on the surface of the substrate, one or more detectors, one or more optical elements to direct the beam of radiation from the detection grating to the one or more detectors, and a processing unit to determine the position of the surface of the substrate on the basis of the beam of radiation received by the one or more detectors. The detection grating and the one or more optical elements are integrated in a single integrated optical element.