To provide a method and apparatus for determining apodization properties of a projection system in a lithographic apparatus.

The method comprises allowing light from a given point in an illumination field to pass through the projection system along at least three different optical paths, and then determining the difference in the intensity of light received in a projection field from the two different optical paths, and calculating apodization properties of the projection system from the intensity difference. It is not necessary to know the intensity distribution in the illumination field. To provide the different optical paths, a pinhole reticle provided with wedges of different orientations is used.