Study Of Precise Location Of Interference Fringe Centerlines Based On The Sub-Pixel In Digital Image

Methods of processing and analysing interference fringes are always being studied as the core of laser interferometric system, and mostly determine the precisionof laser interferometric system. In order to reach the precision level of micron(μm),even nanometer(nm), interference fringes need to be subdivided, and the subdivision level determine measuring precision. Among fringe-subdivision methods, computer software method need not to touch an object and change parts of any apparatus in measuring system when working, so it has the advantages of high speed, low cost and high measuring precision. And Michelson interferometer, as the prototype of many modern interferometers, has simple structure and high precision, it is a typical interferometer and used widely.For this, a digital image processing system of laser interference measuring based on Michelson interferometer was designed in this dissertation, and the interference fringes were subdivided by a image processing software written with Visual C++ to enhance measuring precision.This dissertation mainly discussed general preprocess, spin filter, extraxting fringe centerlines, sub-pixel location, and the method of working out the fractional displacement of fringes, and experiment results show that the method of processing interference fringes proposed in this dissertation has the characteristics of little breaking, little impacted by noise and processing fast. The main work in this dissertation include some points as follows:1. makeing improvement to spin filter, first preprocessing the fringes with Butterworth low-pass filter, then average filtering at a tangent.2. Using edge detection to getting edges of black-white fringes which figured the sign of derivative of intensity variations in interference fringes. this can avoid man-made value for some special points, and enhance the measuring speed with the whole work done automatically.3. Using cubic spline interpolation to fit intensity variations in interference fringes, and getting the fringe skeletons with high precision by finding the maximum or minimum. Then figure out the fractional displacement of fringes.Furthermore, all types of possible error in which bringed by the system software and hardware are analyzed, the actual measurement results are presented and means to reduce or eliminate the errors are suggested.