1. A processor-based method of determining actual properties of a film stack, the method comprising the steps of: directing an incident beam of light towards the film stack, such that the incident beam of light is reflected from the film stack as a reflected beam of light, measuring actual properties of the reflected beam of light, estimating properties of the film stack, solving a mathematical model of the film stack with the estimated properties of the film stack to yield theoretical properties of the reflected beam of light, using the processor, where the mathematical model is solved in part using a fast Z-matrix algorithm, comparing the theoretical properties of the reflected beam of light to the actual properties of the reflected beam of light to yield a cost function, iteratively adjusting the estimated properties of the film stack and solving the mathematical model until the cost function is within a desired tolerance, and reporting the estimated properties of the film stack as the actual properties of the film stack.