We present here the formulae that relate the volume scattering and absorption coefficients of an optically thick extended and horizontally uniform layer to hemispherical fluxes of solar irradiance inside it and allow for strong vertical inhomogeneity of the layer. These formulae, and those from other studies, were applied to processing the results of spectral radiation measurements inside stratus clouds made during three airborne experiments. The spectral dependences of optical parameters of separate cloud layers were determined.