Level sensor and lithographic apparatus

The invention provides a level sensor to measure a position of a surface of a substrate. The level sensor comprises a projection unit arranged to direct a beam of radiation to the surface of the substrate and a detection unit, a detection unit which comprises a detection grating arranged to receive the beam of radiation reflected on the surface of the substrate, one or more detectors, one or moreoptical 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. 本发明提供了一种用于测量衬底的表面的位置的水平传感器，包括：投影单元，被布置成将辐射束引导至所述衬底的所述表面；和检测单元。所述检测单元包括：检测光栅，被布置成接收在所述衬底的所述表面上反射的辐射束；一个或更多个检测器；一个或更多个光学元件，用以将所述辐射束从所述检测光栅引导至所述一个或更多个检测器；以及处理单元，用以基于由所述一个或更多个检测器所接收到的辐射束来确定所述衬底的所述表面的位置。所述检测光栅和所述一个或更多个光学元件被集成在单个集成光学元件中。