Quantitative Assessment of Surface Roughness Based on Defocused Laser Speckle Imaging

The microstructure of a specimen is often characterized by surface roughness during manufacturing and its accurate measurement is the cornerstone of the machining operations. The rapid evaluation of optical technology spurred the nondestructive measurement of surface roughness with high accuracy and high dynamic range. The laser speckle imaging (LSI) technique, on account of its principle of operation, received impetus due to its high sensitivity to surface roughness. The present work introduces the defocused LSI technique for the surface roughness measurement which has not been discussed to date. With defocused imaging configuration, the specimens were found to produce a distinguishable contour of the speckle distribution seemingly representative of the surface roughness. This feature is utilized to establish a quantitative relation between the specimen roughness and the captured speckle distribution. The results show a linear relation between the specimen roughness and the corresponding speckle distribution, for a wide range of surface roughness. The proposed method is robust to the sampling positions of the specimen for a fixed object-to-sensor plane distance. Furthermore, the critical aspects of the proposed configuration are highlighted in comparison to the focused and lensless speckle imaging system. The whole field roughness information can be obtained without additional beam expansion optics. The optical configuration is simple and cost-effective leveraging the potential for in situ surface roughness measurement.