Whole-field determination of surface roughness by speckle correlation

A whole-field method of double-exposure speckle photography is employed to determine metal surface roughness by correlation between two speckle patterns. A movable rectangular aperture that is mounted before an image lens is shifted between the exposures, which results in a decrease in the contrast of the reconstructed Young's fringes with increasing roughness. The technique permits evaluation of the roughness of particular points on a surface as well as the average roughness of an entire surface. Four sets of random surfaces that were prepared by different machine-finishing processes and with roughnesses ranging from 0.6 to 13 µm have been tested. Different methods have been carried out to process the test data, and a practical method for the evaluation of surface roughness is proposed.