The principles for judging inhomogeneity of refractive index of thin film from Δ- Ψ trace spreading over several. periods are given. The experimental results show, that CeF3, LaF3, NdF3 films are essentially homogeneous and ZnS film deposited upon heat substrate has not only two surface layers but also a central layer with gradully enhances index. The distribution and value of errors, when measuring index and thickness of the film by normal ellipsometry, are also discussed.