The determination of the optical constants of absorbing films, particularly on opaque substrates, is a difficult problem when solely using spectroscopic ellipsometry. First, unwanted backside reflections are incoherent with the desired reflection from the front side, which makes the fitting of optical constants difficult. Second, the optical constants of substrate must be carefully characterized in advance, as any small absorption in the substrate would be mixed into the film’s overall optical constants. Third, thickness and optical constants are strongly correlated with each other, which may prevent a unique solution for absorbing films. For the above reasons, quartz, glass slide, cover glass and float glass substrates are studied. Backside reflections of the substrates are suppressed by index matching technique. The results show that the simple technique works well for substrate materials with refractive index in a range from 1.43 to 1.64, including materials such as fused silica, float glass, etc. in a spectral range from 190 nm to 1700 nm. The refractive index and extinction coefficient of the substrate are fitted by ellipsometricψdata and the normal spectral transmittance T0. The results are consistent with the literature reported. Finally, a Combined ellipsometry and transmission approach is used to determine the thickness values and optical constants of the diamond-like carbon (DLC) film coated on the quartz and the amorphous silicon (a-Si) film coated on the glass slide and cover glass accurately.