Using the matrix technique for the evaluation of the optical properties of thin dielectric layers it is shown that optical measurements (with present accuracies) are unable to distinguish between a homogeneous model, a three-layer model or models with a higher number of layers for the secondary black foam films. This fact is used to develop a simplified method of extracting the film parameters corresponding to a symmetric 3-layer model from the optical data available. The technique is applied to the results of Corkill et al. on the equimolar sodium decyl sulphate+decyl trimethylammonium bromide system. Finally the effective extinction coefficients in very thin films can be very different from those of thick films because of optical interference effects related to the refractive index. For water films in air the errors involved may be as large as ∼33 %.