A nonstandard photolithographic exposure tool motivates the search for a method to determine photoresist thickness in real time. Optical physics of two-interface thin film systems and the theory of Fourier series reveal a way to calculate photoresist thickness by applying an FFT (fast Fourier transform) algorithm to the reflectance spectrum of a light beam incident on the photoresist. Analyses of simulated data and preliminary measurements assess the speed and accuracy of the FFT algorithm and suggest further areas of research.