Silicon germanium (Si1-xGex) is a material with high mobility and relatively low bandgap making it an attractive candidate for the bottom subcell in a multi-junction solar cell. Optical constants of S1-xGex films grown on silicon substrates with germanium (Ge) compositions of 77%, 82%, 85% and 88% from wavelength of 400 nm to 1450 nm at room temperature are reported. Spectroscopic ellipsometry is used to obtain the real part of refractive index for the whole wavelength range and the extinction coefficients for short wavelengths. Optical transmittance ratio of two structures being different only in the thicknesses of the Si1-xGex films is used to extract the extinction coefficients for long wavelengths. We demonstrate that the optical constants of Si1-xGex films grown on silicon substrates with the aid of graded buffer layers are similar to that of their corresponding bulk materials. However, the absorption coefficients of S1-xGex with 88% Ge determined by us at around 1000 nm is three times higher than previously interpolated results. The main reason for this discrepancy may be due to the inaccuracy of interpolation in the wavelength range where the material's bandgap changes very quickly. In addition, it should be noted that the Ge composition of the Si1-xGex films we are able to obtain are determined to be +/- 1% which is much more accurate than the previous experimentally determined results which have an accuracy of +/- 5%. (C) 2015 Elsevier B.V. All rights reserved.