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3D Roughness Metrology

Working Group 5.14


Roughness metrology for quality assurance has traditionally been realized by tactile probing with profilers. Profilewise scanning requires larger measurement times than optical and areal measurment systems. Furthermore, the mechanical contact of a probe tip may cause damage depending on the properties of a material. Therefore, instrument manufacturers have driven progress of the development of optical topography metrology. To industrial users the issue roughness metrology is of vital importance, in particular with regard to the functionality of machine parts. Optical methods to measure roughness and texture on machine parts are more challenging than tactile methods, as results may differ depending on the specifics of the measurement principle of an optical device. Working group 5.14 takes these challenges by performing investigations and developments on characterization and traceability of optical topography measurement procedures. This includes the development of appropriate material measures.

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Development of Optical Procedures to Measure Texture and Topography

Working group 3D roughness metrology supports the development of optical procedures to measure texture and topography by

  1. developing material measures that serve to characterize topography measuring microscopes, respectively by participating in projects accordingly;
  2. developing guidelines and ISO standards or contributing to the developing process;
  3. implementing procedures to analyse areal topography data according to the specifications in standards of the ISO 16610-series (filtration) and according to the standard defining roughness parameters,Opens external link in new window ISO 25178-Serie;
  4. calculation of simplified models of the optical mapping process to better understand the response behavior, band limitation and the fidelity of the signal and comparing these with experimental observations;
  5. organizing and carrying out comparisons of measurements and analysis procedures with other National Institutes as well as with instrument manufacturers with regard to the characterization of instruments by means of material measures.

The central issue of all applications, i.e. the measurement of microstructured topographies and textures (roughness), is concerning the signal fidelity, which is referred to as topography fidelity. Does an instrument reproduce a topography reliably within a certain accuracy class? Which size and which type of geometry do the smallest features have that are still transferred correctly by a measurement process? To answer this question respectively to specify the bandwith of a dimensional measurement task, vertical and lateral resolution need to be regarded in combination. Therefore, the aspect ratio of structures together with their slopes and curvatures need to be incorporated to the criterium structure resolution. To investigate structure resolution, various well-defined structures are realized as material measures.  There are two different categories of material measures, those with features of defined, regular geometries as for example sinusoidal structures, steps and line gratings of rectangular profile, pyramids with terrasses, or those objects with a topography consisting of stochastic respectively pseudo-random structures.
Following manufacturing processes are used:

Diamond turning (cooperations with working group 5.56 Manufacturing Technology)

By using a face turning method of an ultra precision lathe the surface of a 100 µm nickel phosphoric coating on a copper disk is structured. Each disk is cut into single segments. The disk has a radius of several ten Millimeters such that the arc segments are almost straight within a typical measurement range. So, they can be regarded as 1D profiles in radial direction.

Surface finishing by a combination of grinding and lapping processes on silicon (cooperations with the company SiMetricS)

Grinding abrases material with the grains fixed to the tool structuring with lay and lapping with rolling grains structuring isotropically. SiMetricS has developed procedures to control lay and isotropy by moving the tools on defined trajectories.

Focused ion beam (FIB) Milling (Cooperation with Pointelectronics)
As part of the project Opens external link in new windowMicroparts of the European grant for metrology EMRP/EMPIR micro geometries have been manufactured (cylinders and spheres) with their surfaces been textured by use of the FIB milling process.This procedure is also employed to manufacture roughness standards for the field of mirco roughness metrology.

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Optical 3D Microscopes for Services

Working group 5.14 is equipped with optical 3D microscopes of various measurement principles, which are employed for service measurements:

  1. Confocal scanning laser microscope (CLSM) Olympus Lext OLS 4100;
  2. Sensofar S-Neox, with a static confocal mode and with different interference microscopic modes (vertical scanning interferometry VSI, phase shifting interferometry PSI, extended PSI);
  3. MicroMap of Atos with different interference microscopic modes (VSI, PSI).

One example is supporting investigations of working group 3.62 Explosion-protected Communications and Sensor Systems on spark ignition caused by separating electrodes, [Uber et al]

The properties of a surface are characterized by a multitude of parameters, the majority of which are defined in the standards for surface metrology. Our working group's task consists in calibrating these parameters for measurement standards used for the calibration of surface measuring devices.
The aim of this working group is to achieve a better comparability of the measurement results obtained when calculating surface parameters.

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Former working group 5.15 now integrated into 5.14

The former working group 5.15 Calibration of roughness parameters has been integrated with all its activities and tasks into working group 5.14 headed by Dr. Sai Gao as of January 1, 2023. Contact persons remain Thorsten Dziomba for calibrations and for the PTB reference software (Opens external link in new windowhttps://www.ptb.de/rptb) Dr. Dorothee Huser.

New standards for roughness measurements by stylus profilometers: ISO 21920

The standards for profile roughness measurement have been revised and brought together in the new three-part series of standards ISO 21920, which was published in the course of 2022 and replaces well-known standards such as e.g. ISO 4287, ISO 4288 and ISO 13565-2 and -3. Calculation rules have been adapted to technical progress and made more precise in many places. Some of the filtering via the ISO 16610 series of standards is newly regulated, for example for the characterization of the contact area of stratified surfaces (so-called Rk parameters), which sometimes lead to significant differences between the old Rk parameters (according to ISO 13565-2 with filtering according to ISO 13565-1) and the new ones (according to ISO 21920-2 with filtering according to ISO 16610-31).
More information Opens internal link in current windowhere.

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