We report surface-induced optical anisotropy (SIOA) spectra and dielectric function data of vicinal Si(001) surfaces using reflectance-difference spectroscopy (RDS) and spectroscopic ellipsometry (SE). To find the main contributions of the optical response we took data for clean and by hydrogen, oxygen, and arsenic modified surfaces. The SIOA lineshapes, as measured by reflectance-difference (RD) spectroscopy, can be either derivative-like or similar to the dielectric function as measured by SE. The derivative-like spectra appear to be associated with step contributions while the dielectric-function-like behavior appears to arise from terrace dimers and/or changes in chemical species bonded to the steps. In addition, we present SE data of silicon nanostructures formed by electrochemical etching and find that their behavior is dominated by interface properties.