A theoretical framework is presented to allow for the determination of the basic structural parameters of a porous silicon thin layer using constructive-destructive positions of Fabry-Perot fringes in air. The structural parameters include film thickness, porosity, and refractive index between 350 and 2000 nm. The model is general and can be applied to determine the optical properties of any thin layer that exhibits a Fabry-Perot fringe pattern. Such information is of importance to fabricate practical sensing devices with low costs for various applications in biomedicine and analytical and environmental chemistry.