how can i obtain optical constants

Ravian

can someone trained in ellipsometer can guide me how can i obtain optical constants of a thin film which is sandwiched between two films of known optical constants. with such multilayer samples interference oscillations become difficult to resolve but i am sure that top layer is non-absorbing (and thin enough to pass light) so light is being reflected from the layer beneath it whose optical constants needs to be known. they bottom layer is substrate.
i think understand the basic theory of ellipsometry and know what is happening. i also know that optical constant can be obtained in this way but not sure how can i resolve interference oscillations to identify which one comes from the top layer which one from my film with unknown optical constants.