Spectroscopic imaging ellipsometry is used to characterize films containing self-assembled SiGe/Si in-plane nanowires grown by molecular beam epitaxy on a Si(001) substrate. The spatial resolution of the order of similar to 1 mu m allows to study individual nanowires. The obtained images consist of both psi and Delta values measured in every pixel. From them, spatially resolved contributions to the dielectric function within the films are well visible, coming for example from composition variations. In particular, a strong in-plane anisotropy of the optical response of the nanowires is evidenced. Spectroscopic measurements from different regions of the film are analyzed and compared to spectra calculated using standard ellipsometry multilayer models. The results show the potential of imaging ellipsometry to study micro- and nanostructured semiconductor samples. (C) 2016 Elsevier B.V. All rights reserved.