STUDY OF LASER AND WHITE-LIGHT SPECKLE METHODS FOR DEFORMATION MEASUREMENT (STATISTICS, CORRELATION, FRINGE, OPTICAL FILTERING, DECORRELATION).

摘要

The use of laser and white light speckle for displacement and deformation measurement has found wide application in mechanics. Traditionally deterministic approach has been adapted to analyze the process. In this dissertation a statistical analysis for these methods is presented resulting in better understanding of the various aspects of these methods. The concept of cross-correlation is invoked in the study of speckle motion in free space. This is followed by a comprehensive statistical analysis of the entire recording and filtering processes of the one beam laser speckle method. It is shown that the amplitude of the information-carrying cosine fringes is modulated by three factors: a decorrelation factor (gamma), a speckle displacement factor (OMEGA), and a speckle displacement gradient factor (psi). With the help of an ambiguity factor, which sets a critical value for the product (gamma)(OMEGA)(psi), a criterion for fringe discernibility is established. The upper limit of the method can then be determined from this criterion. The statistical analysis is then extended to the white light speckle method, revealing several important differences between these two methods in aspects, such as speckle displacement, halo function and decorrelation factor. The error in positioning a filter in the transform plane changes isothetic fringe pattern in whole-field filtering. An analysis based on a "twin speckle" model leads to a fringe equation, which can be used for error estimation of fringe order determination. Many experiments were carried out to substantiate the claims of theoretical analyses.