SPECKLE METHOD FOR WHOLE FIELD SURFACE ROUGHNESS MEASUREMENT

Abstract

In this thesis, a combination of two whole field speckle methods is proposed for the evaluation of roughness parameters of flat metallic surfaces with random microscopic height distributions. A subjective speckle pattern of the object surface is recorded on a holographic film using either the single exposure technique or the double exposure technique. Roughness information is subsequently retrieved from the diffraction pattern obtained from either a pointwise or a whole field filtering process. The major findings of this project has been sent for publication*. The measurement of the intensity transmittance of a single exposure specklegram using the pointwise filtering allows point by point evaluation of the values of the mean slope factor and the RMS roughness of a rough surface, whilst using the whole field filtering allows the evaluation of the average values of the two parameters for the entire surface. By translating a mask with an aperture that is mounted in front of the image lens between exposures in recording a double exposure specklegram, a method of the speckle correlation measurement is proposed for determining the RMS roughness of the test surface. The RMS roughness is evaluated point by point via pointwise filtering whilst the average RMS roughness is evaluated via whole field filtering. The measurement of the intensity transmittance can be adapted to doubly exposed misfocused image specklegrams which, on combining with the speckle correlation measurement, is capable of determining the RMS roughness and the surface slope factor. With both vertical and horizontal roughness values determined, a random rough surface can be expressed completely. This thesis is divided into five chapters: Chapter I introduces some of the general phenomena of speckle; clarifies some of the terms used in optical physics; and explains the influence of surface roughness on the speckle patterns. Chapter 2 classifies the existing methods of surface roughness measurements and reviews the relevant literature; outlines the objectives and scope of the present research and highlights a well-accepted scattering model for the rough surface. Chapter 3 develops the theoretical background of the present project. The investigation of the intensity transmittance of specklegrams is based on the Beckmann-Spizzichino solution of rough surface scattering and the principle of geometrical optics. The mathematical derivation for doubly exposed misfocused image speckle correlation is based on the Fourier transform analysis of the optical system and statistical analysis of the rough surface. The relationship between the contrast of fringes, which is a measure of the speckle correlation, and the RMS roughness is carried out. Chapter 4 describes the experimental arrangements and experimental results. Applications using the proposed methods to determine a rough surface completely are demonstrated. By measurements of the intensity transmittance and the extent of the autocorrelation of a specklegram, vertical roughness information in both vertical and horizontal directions can be retrieved from one doubly exposed specklegram recorded at directions not far away from the specular reflecting direction of the surface. Finally, Chapter 5 gives a conclusion of the present study. It provides a summary of accomplishments of the present project and suggests some improvements and adaptations for future work in roughness determination.