Speckle correlation for the analysis of random processes at rough surfaces

The importance of technological processes like corrosion, ablation or depos ition causes interest in the quantitative monitoring of changes at rough su rfaces. Thus, there is a need for effective methods to measure the statisti cal parameters characterizing changes in the profile or the material compos ition of such objects. The speckle field scattered from the surface is used as information carrier and its change is measured by correlation. This is realized by sophisticated data acquisition and digital processing technique s. An important issue is the interpretation of the correlation output in te rms of statistical parameters describing the surface change. For many rando m surfaces a geometrical relation between surface profile and optical phase proves satisfactory. This allows to determine the standard deviation of th e profile change. For a verification, speckle decorrelation in model surfac es of known deviation is measured. The paper introduces the speckle correla tion concept, outlines some history and current setups and describes method s for data evaluation. The reliability of the quantitative interpretation o f the speckle decorrelation is demonstrated. The method is illustrated by s tudies of metal corrosion and material removal in the cleaning of historica l objects by laser ablation. (C) 2000 Elsevier Science Ltd. All rights rese rved.