In the case of weak absorption, the spectral reflectance of a thin film on a nonabsorbing substrate can be used to determine the spectral dependence of the absorption constant. For this purpose, a relatively simple method is described evaluating the envelopes of the interference extrema. This method as well as a Kramers‐Kronig analysis were applied to epitaxial, polycrystal‐ line, and amorphous silicon films in the energy range 0.5–5.5 eV. With decreasing crystal quality, changes in the spectral dependence of the optical constants of these films are observed. This can be understood in terms of a decreasing long range order.