The invention relates to a thin-film interferometer, in particular destined for chemical or biochemical measuring and analysis systems, comprising a planar waveguide in the form of an integrated optical sensor element which comprises a thin, light guiding film, partially coated with a covering layer. According to the invention, the light guiding film of the interferometer forms at least one measuring and reference section for light guided in the film and light modes of the light are interferable after passing through the measuring and reference section within the light guiding film.