The combined optical method enabling us to perform the complete optical characterisation of weakly absorbing non-uniform thin films is described. This method is based on the combination of standard variable angle spectroscopic ellipsometry, standard spectroscopic reflectometry at near normal incidence and spectroscopic imaging reflectometry applied at normal incidence. The spectral dependences of the optical constants are determined using the non-imaging methods by using the dispersion model based on parametrisation of the density of electronic states. The local thickness distribution is then determined by imaging reflectometry. The method is illustrated by means of the complete optical characterisation of SiOxCyHz thin films. (C) 2011 Elsevier B.V. All rights reserved.