A closed-form formula for the film optical constant is presented. The derivation of the formula itself is not presented to save the reader lots of involved transformations and algebra. The formula in itself is algebraically accurate and does not introduce errors. The effects of experimental errors, random and systematic, are presented. The results are very accurate. The inversion process is very fast, stable, and resilient, does not need a guessed close-to-solution or any starting value, always provides the correct answer with no divergence in any case, and is not iterative in nature. Clearly, those are important advantages over the widely used, manufacturer supplied, fitting routines. It provides for real-time applications in research and industry.