The present paper considers the calculation of the absorption coefficient, the index of refraction, and the thickness of thin films using optical transmission data only. To solve the problem, a pointwise constrained optimization approach is used. The method consists of defining a nonlinear programming problem, efficients to be estimated with linear constraints that represent prior I;knowledge about the physical solution. Numerical experiments using artificially generated transmission data show that the method is reliable, even in cases where no interference fringes exist. Examples are presented and discussed.