To detect whether a substrate is in a focal plane of a scatterometer, a cross-sectional area of radiation above a certain intensity value is detected both in front of and behind a back focal plane of the optical system of the scatterometer. The detection positions in front of and behind the back focal plane should desirably be equidistant from the back focal plane along the path of the radiation redirected from the substrate so that a simple comparison may determine whether the substrate is in the focal plane of the scatterometer.