a system for characterizing geometric structures formed on a sample on a real time basis is disclosed . a multi - parameter measurement module generates output signals as a function of either wavelength or angle of incidence . the output signals are supplied to a parallel processor . the processor creates an initial theoretical model and then calculates the theoretical optical response of that sample . the calculated optical response is compared to measured values . based on the comparison , the model configuration is modified to be closer to the actual measured structure . the processor recalculates the optical response of the modified model and compares the result to the measured data . this process is repeated in an iterative manner until a best fit is achieved . the steps of calculating the optical response of the model is distributed to the processors as a function of wavelength or angle of incidence so these calculations can be performed in parallel .