This article has two major purposes. The first is to demonstrate the utility of the invariant method for the analysis of the optical properties of thin films. The use of invariants allows the determination of a maximum of information on the layer, independent of arbitrary optical models. In particular, it is possible to determine the surface concentration and an average thickness of the adsorbed layer, with additional information about the distribution of matter in the layer. The limitations of the invariant method are discussed in detail. Second, the optical properties of an antigen/antibody (both immunoglobulin G proteins) film adsorbed at the water/silica surface are explored, comparing the results of the invariant method of analysis and those of optical models, in particular the uniform isotropic layer and the bilayer. The results consistently indicate a depletion layer near the solid interface.