Surface properties can influence not only the haptics and aesthetics, but also the mechanical, electrical or chemical behavior of products. Information on flatness or roughness is therefore an important basis for optimization. They can be used, for example, to increase or reduce friction, minimize wear, increase resistance to external influences or improve transmission capacity. As the surface is often the result of a multi-stage manufacturing process, only a carefully coordinated and quality-monitored production process can lead to the desired result. Optical measuring methods as a non-contact and non-destructive analysis and testing method open up interesting possibilities here, as they can be used for almost all materials and are also suitable for sensitive surfaces.

In surface measurement, roughness, form and waviness are not sharply delineated characteristics that exist separately from one another. Instead, a surface can be described as a superposition of numerous wavelengths, whereby the transition from the particularly long-wave form components to the waviness components to the short-wave roughness components is fluid. Frequency filters are responsible for the separation. By applying these low-pass or high-pass filters with Gaussian characteristics, a bandwidth-limited profile or a bandwidth-limited surface is then available for further evaluation. The selection of the respective cut-off wavelengths is of central importance, as different measured values for the desired measured variable can result depending on the setting.

The measurement chains for surface or profile-based evaluation, which can be applied to optical measurement technology, are described in the ISO 25178 and ISO 4287 series of standards. In the case of profile measurement, the cut-off wavelength, the individual measuring distance and the evaluation length are determined as a function of the surface properties on the basis of a table. For this purpose, the expected texture parameters are first estimated and then test measurements are carried out (this procedure will soon be modified by the new standard DIN EN ISO 21920). There is no comparable table for area measurements, but it is recommended to use the same or similar values as a basis for test measurements. The amplitude and height parameters frequently used in practice have largely been extended in the newer standard to include areal evaluation. The advantage of area-based measurement and evaluation of the topography is that it does not depend on the choice of measurement position and therefore provides more reliable results, especially for inhomogeneous or faulty surfaces.

Characteristics in surface metrology