Requirements for the application of speckle correlation techniques to on-line inspection of surface roughness

Conventional surface roughness measurements using profilometric instruments (either mechanical or optical) are standardized and extensively applied in industry. However, these techniques are not suitable to an on-line inspection of industrial machining processes. If an optically rough surface leading to a diffuse scattering is illuminated by coherent light, a speckle pattern occurs showing a granular spatial intensity distribution of the scattered light. In general, the averaged intensity distribution depends on the distribution of the local surface slopes. However, if two speckle patterns at different laser wavelengths are compared via correlation techniques, a measure of the root-mean-square (rms) roughness (Rq) can be obtained, based on certain statistical assumptions. This paper contributes to the application of speckle correlation techniques to the inspection of machined surfaces. Surface statistics dependent on the manufacturing processes are checked. Optical arrangements of a measuring device are introduced. Furthermore, results showing the limits of speckle correlation are presented.