A new optical method of rms surface roughness (Rq) measurement for diffusely-reflecting or scattering surfaces with Rq greater than 1 micron is disclosed, and its application to grit- blasted and plasma-sprayed surfaces described. Data acquisition by the 'multiple line shadow' (MLS) method over a small area (typically 4 mm2), image analysis, and Rq readout require at most 10 seconds, with standard deviations of 10% of mean value attainable. Measurements are made by hand-held or automated optical probe at the workpiece, with no necessity to remove the piece from its production jig. The MLS method is absolute; no calibrations are required. When automated, the MLS probe need not touch the nominally flat or large-radius workpiece surface. Minor probe and algorithm modifications allow non-contact measurement of step heights or coating thicknesses in the 1 - 30 mil range (.025 - .75 mm), also in times of 10 seconds or less at the workpiece.
John N. Pike,
"Rapid surface roughness measurements of coarse objects", Proc. SPIE 1776, Interferometry: Surface Characterization and Testing, (15 December 1992); doi: 10.1117/12.139243; https://doi.org/10.1117/12.139243