Portable device for measuring surface roughness in-situ with a non-invasive optical technique

The instrument is based on a commercial laser probe using so-called conoscopic holography. This non-contact technique allows the measurement of distances with a very high resolution. The probe works by measuring the spatial frequency of fringes in an interferogram formed by imaging a laser focus through a uniaxial birefringent crystal. This frequency changes depending on the through-focus distance of the surface where the laser is focused. The probe we are using implements a parallel measurement technique which allows to obtain several points in a line for a single measurement, thus giving a micro profile of the surface. By changing the probe lens, different height resolutions can be obtained. As the device is almost insensitive to surface reflectivity, it is particularly recommended for those surfaces showing high chromatic variation. We added to the probe a scanning device to obtain a three-dimensional (3D) micro map of a small area, where the surface roughness can be measured. We tried a scanning scheme with a rotating plane mirror. The micro 3D maps yield all information needed to calculate the roughness, and much more. They are an accurate representation of the 3D shape of the measured area. The instrument was validated on an ancient calcareous stone column with fine details engraved.