Roughness And Correlation-Length Measurements Of Rough Surface Objects Using The Speckle Contrast In The Diffraction Field

Abstract

A method using the speckle contrast for the determination of both the roughness and the correlation length of surface height variations of a rough surface object is studied on the basis of the statistical properties of the Gaussian speckle field in the diffraction region. The speckle contrast is theoretically derived as a function of the illuminating condition for the object and the surface parameters of the object. It is found that the roughness depends almost only on the ratio of the two speckle contrasts obtained under the two different illuminating conditions for the object. By solving the equations expressing these two speckle contrasts, it is also found that an unique solution exists for the roughness and the correlation length. Therefore, the correlation length can also be determined from the two speckle contrasts. Experiments are performed to verify the present method. This method has an advantage of the simplicity in a measuring procedure where only two speckle contrasts are required to determine the surface parameters.