In an angularly resolved scatterometer, an aperture plate including at least one obscuration extending into the image of the pupil plane is provided. Defocus values of a target pattern are determined from the radial distance between the innermost point of the images of the obscurations and the nominal center if the pupil image. Defocus errors are compensated for by capturing a plurality of normalization images using a reference plate at a plurality of different defocus positions and subtracting a suitable normalization from the measurement spectrum of a target pattern.