Autocorrelation analysis of spectral dependency of surface roughness speckle patterns

When a surface is illuminated with a highly coherent light such as a laser beam, the speckle pattern of bright and dark regions is observed. It depends on the surface parameters and carries important information about the roughness of the surface. Various methods and techniques are employed for the determination of surface roughness parameters from speckle pattern properties. In this paper, an experimental approach for surface roughness evaluation based on the autocorrelation analysis of the spectral properties of speckle patterns caused by milled metal surfaces is reported. The speckles at three 633, 604 and 543 nm wavelengths of He-Ne laser were analyzed. It was found that autocorrelation analysis is very sensitive to small variations in speckle sizes, caused by spectral properties of speckle patterns such as increasing the wavelength lead to increased speckle sizes. The results are in good agreement with the results obtained from the mechanical stylus profilometer for the milled metal surfaces with roughness values Ra; 0.36µm (low roughness) and 1.98µm (high roughness). The technique reported here has a great potential for precise and non-contact optical measurements of rough surfaces.