The present invention determines property of a target (30) on a substrate (W), such as a grating on a wafer. An inspection apparatus has an illumination source (702, 710) with two or more illumination beams (716, 716', 716", 716"') in the pupil plane of a high numerical aperture objective lens (L3). The substrate and target are illuminated via the objective lens from different angles of incidence with respect to the plane of the substrate. In the case of four illumination beams, a quad wedge optical device (QW) is used to separately redirect diffraction orders of radiation scattered from the substrate and separates diffraction orders from the two or more illumination beams. For example four 0 th diffraction orders are separated for four incident directions. After capture in multimode fibers (MF), spectrometers (S1- S4) are used to measure the intensity of the separately redirected 0 th diffraction orders as a function of wavelength. This may then be used in determining a property of a target.