Laser speckle roughness measurement of grit-blasted surfaces

The surface roughness of grit-blasted steel specimens has been measured using a non-contact method based on the correlation of speckle images derived at different incidence angles. Measurements of roughness using the speckle method agreed to within ∼±1.5 μm with independently measured average surface roughness derived using a surface profilometer.

It was noted that although the speckle analysis predicted RMS roughness, better agreement with reference roughness measurements was obtained with the speckle measurements of average roughness.

It was shown that errors in the measurement accuracy arose from lens aberrations, image miscorrelation due to the finite pixel size relative to the speckle size and shadowing of the laser beam by the surface. A sub-pixel image correlation method was demonstrated for the correction of errors associated with pixel miscorrelation.