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 speckles at three 633, 604 and 543 nm wavelengths of He-Ne laser were analyzed. 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. Various methods and techniques are employed for the determination of surface roughness parameters from speckle pattern properties. It depends on the surface parameters and carries important information about the roughness of the surface. 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. If this examination reveals that the imaged speckle intensity image is not an unbiased estimator of the underlying random speckle field, then adjustments should be made to ensure that the images taken are unbiased estimators of the true speckle field. Finally, it is recommended that when using speckle techniques such as laser speckle contrast imaging, investigators should examine the first and second order spatial statistics of a speckle image prior to collecting actual data. Specifically, this study examines the effects of first and second order spatial statistics of speckle intensity images on laser speckle contrast imaging results. If this condition is not met, the quality of the results will be compromised. Herein, we demonstrate that when sampling (imaging) speckle patterns, it is critical that the sampled speckle patterns are unbiased estimators of the underlying random speckle field. Laser speckle is a statistical phenomenon and should be treated as such when establishing experimental parameters for using laser speckle techniques to infer information about the dynamics of a coherently illuminated medium, such as biological tissue.

Speckle phenomena in optics