High performance devices often require good quality layers, homogeneous composition and crystallinity, then the homogeneity in depth of the layers is of major interest. Our study investigates the possibility of evaluating the inhomogeneity of the refractive index of transplant films deposited on absorbing substrate by multiple-angle ellipsometry. We propose a method which is based upon the determination of the best experimental conditions leading to thr minimal uncertainty one can expect on the physical parameters to be determined. A theoretical study of an inhomogeneous Al2O3 layer deposited on an InP substrate is presented as an example.