The uniform thin-film model has often been used to deduce film characteristics from optical data,even when the film is assumed to consist of discrete elements, for example surfactant micelles. Here we test the uniform homogeneous film model on optical data from films known to be particulate, consisting of polystyrene latex spheres adsorbed at the liquid/silica interface : We find that in this case, the uniform film model gives the characteristics of the film accurately for films of thicknesses of less than 100 nm. For thicker films, deviations occur.