We present ellipsometric investigations performed within the spectral range 2 less than or equal to <(h)over bar omega> less than or equal to 4 eV on samples with film thicknesses h below 30 nm. The calculated thicknesses are dependent on the supposed model and on the wavelength. The discussion shows the sensitivity of h against the variation of the input data and throws some light on difficulties with this essential parameter for nanoscale devices.