dThe composition and surface roughness of thin Si1-xGex films grown on Si(100) can be evaluated in-situ by using the simple procedure described here for analyzing dielectric functions. By using the Lorenzian-type reference dispersion relation and a minimum film thickness until saturation of the shape around the E-1 critical point, the E-1 energy is determined by line-shape analysis. The Ge composition x derived from the E-1 energy agreed, with uncertainties of +/-0.05, with values in the literature. The correspondence between atomic images obtained by transmission electron microscopy and the surface roughness layer height obtained from linear regression analysis of the dielectric function is discussed.