We demonstrate a direct measurement of thickness gradients in vertical soap films with a resonant differential interferometer, i.e., the Jamin-Fabry-Perot interferometer. Two regimes are investigated: thick colored films with gravity- and capillarity-induced gradients, and silvery-gray to common black films which are quasi-independent of gravity. In the colored zone, our differential method is an ideal tool with which to isolate the large thickness instabilities of the film reaching that characterize the end of its drainage. Using the so-called law of such an interferometer, thermal-induced thickness variations as small as are isolated in the gradient-free common black film.
Actually, due to small constructive interferences in the film, the effective thickness is equal to , with for a film of water.