A zero OPD location algorithm for WLI based on the symmetry criterion

A symmetry criterion is designed according to the irradiance distribution characteristic of White Light Interference (WLI). A zero Optical Path Difference (OPD) location algorithm is presented based on the symmetry criterion and discreteness of sample. The location algorithm consists of two steps: 1) Using the symmetry criterion and its variation trend to search and locate the measurement sub-peak around the maximal value in the data, which is the closest to the zero OPD; 2) Using fitting method to further locate the zero OPD and peak value based on the located sub-peak in the step 1. The location validity can also be evaluated by the symmetry criterion. The experiment of data processing and algorithm comparison shows that this location algorithm has advantage of discriminating and pick up zero OPD, which can meet the measurement need of the high precision WLI instrument.