Spectroscopic ellipsometry of thin films frequently encounter the problem of multi-valued solutions in a regression fit. A good starting thickness value is critical to reach the correct solution. We present a formulation to extract optical parameters from film-substrate systems and spectroscopic ellipsometry independently of the film thickness d. We do not need to include it as a parameter in a regression fit. The basic idea is to use the envelopes of tan curves instead of the curves itself. This approach simplifies the analysis efficiently. The envelope analysis is thickness-independent. As we have two envelopes, we may extract two optical constants, and by also using the points touching the envelopes (the envelope-extremes) we may extract the thickness separately. We develop and apply the formulation for non-absorbing media and we also check its validity for absorbing substrates. Another important conclusion derived from the results is that the refractive index of the film can be obtained even if the substrate is unknown. (C) 2002 Elsevier Science B.V. All rights reserved.