In an inspection apparatus, a target on the surface is illuminated with illuminating radiation that comprises first and second illuminating components. The illuminating components form one or more periodic illuminating patterns on the surface. A plurality of scattered radiation patterns formed by the illuminating radiation after scattering by the target is captured at a detector for a number of values of a controllable characteristic of at least one of the illuminating components. The captured radiation is then used to reconstruct data describing the target.