Abstract: The present invention provides a method for determining a displacement in a z-direction, of an object which is fixed in a holder of an apparatus, by means of an interferometer system, wherein the object is illuminated by a beam of radiation, said beam being provided by said apparatus and having an optical axis extending in the z-direction. The method comprises arranging the measuring mirror(s) and/or measuring laser beam of an interferometer system such that no relevant part of the laser beam is parallel to the z-direction. This ensures that the interferometer system and its parts may be arranged away from the beam of radiation, allowing larger diameter projection systems for said beam of radiation, as well as more homogeneous air showers around said object. Thus the quality of the illumination of the object may be improved.

Abstract: A lithographic apparatus includes a measurement system including at least one optical component and at least one electrical component. The electrical component is configured to dissipate heat. The optical component is mounted on a first frame of the apparatus, and the electrical component is mounted on a second frame of the apparatus that is thermally and mechanically decoupled from the first frame. An optical coupling is provided between the first frame and the second frame.