Measure surface roughness optically
Surface topography and its structural details like surface roughness is traditionally inspected using tactile stylus-based profilometers (stylus instruments) by guiding a probe tip over a workpiece surface and detecting each vertical deflection by the surface texture. With this method, the information about the surface topography including surface details like roughness is obtained along a 2D profile. It is questionable to which extent profile-based information could describe surface characteristics and provide function-oriented information.
Optical vs tactile surface roughness and texture measurement
If the surface has randomly distributed features, the result for the roughness parameter is strongly influenced by the measuring position. In many cases, profile-based surface description is insufficient to provide information about functional behavior of the surface. Profile based surface characterization allows only limited information about the cause of poor functionality and so includes limited information for quality control purposes.
Areal 3D surface roughness measurement data
Three-dimensional, areal topography measurement is not subjected to those limitations. Not only does it provide an image of the surface that makes it easier to understand, but areal measurements also enable a function- and structure-oriented evaluation. Furthermore, 2D profiles can easily be extracted from the areal measurement data which can again be evaluated according to the common rules of profile-based roughness evaluation. In contrast to a tactile roughness measurement, optical 3D roughness measurement is non-contact and non-reactive thus avoiding any damage or influence on sensitive surfaces by the measurement procedure.
Areal surface roughness measurement data provides an easy and complete view of an entire surface. In contrast, a profile measurement contains only a limited section of the entire surface and is less intuitive.
ISO for profile and areal surface evaluation
The measurement chains for the areal or profile-based surface evaluation are described in ISO 25178 or ISO 4287, they differ from each other with some details.