Optical metrology in a new light

Many classic industries such as the automotive industry and its suppliers or the tool construction and mechanical engineering industry look upon optical measurement technology with a sceptical eye – and rightly so. Often, it is not clear how the measuring points were generated with this method. Confovis follows the strategy of guaranteeing the industrial user maximum transparency during data acquisition. The system noise and resolution are specified in accordance with VDI 2655.
In addition, Confovis meets the concerns of customer scepticism by providing a quality value for each and every measurement point. With automated multiple technical measurements, the measurement equipment capability for the intended use in a production process can be specified using the Cg and Cgk values by performing the inspection on a suitable comparison standard and then on the work piece to be measured. 

Comparison of optical and tactile measurement principles

So far, the stylus profilometers has been suitable for most of the requirements listed here. Modern surfaces created through finishing, lapping or coating require surface measuring for identifying randomly distributed structure elements. This can be achieved with the combined measuring principle provided by Confovis.

3D measurement – for more complete surface information

The functional parameters of a surface are critical for determining the properties of a product. So far, these parameters have been measured using classical techniques such as tactile profilometers etc. However, the 2D measurements they make along limited profile lines no longer meet the modern standards for a complete assessment of functional surfaces. Confovis therefore measures the entire surface in order to produce comprehensive surface data by means of 3D measurement. This is especially important when analysing structures distributed randomly over the surface for the purposes of quality control.

A surface with randomly distributed structures was measured using the Confovis system and its topography depicted as a 3D image. The surface data thus obtained provide a basis for comprehensive analyses. For comparison, two example profile lines were also placed on the surface and their total profile depth (Pt) measured. Areal roughness values measured according to DIN EN ISO 25178 allow more assertions to be made about the surface quality.

Roughness measurements verifiable by means of standards

For the measurement of minuscule roughness, Confovis recommends using one of the roughness comparison standards certified by the Federal Physical/Technical Institute (Physikalisch Technische Bundesanstalt) such as the superfine Halle KNT 4070-03. With this comparison standard, not only are the arithmetic mean roughness (Ra) and the average roughness depth (Rz) output, but also the core roughness (Rk) with its pertinent ridge and crest values (Rpk and Rvk respectively). These value are relevant for functional surfaces. For measuring angles and radii, Confovis recommends using comparison standards which are as similar as possible to the structures to be measured.
In the following, the roughness standard Halle KNT 4070-03 was measured with a Confovis measurment system. Firstly, the profile curve and the parameters from the calibration certificate are shown. Below, the results from the Confovis measurements are shown.

Minimum artefacts - thanks to the patented measuring method

Until now, these surface parameters have been detected by means of traditional measuring processes, such as tactile systems. Confocal measuring processes with laser, focus variation and white-light interferometry are reaching their physical limits. The laser causes coherence and speckle effects. The focus variation reaches its limits for reflective and fine surfaces because of the low axial resolution. Due to low acceptance angles, imprecise results are achieved when using the white-light interferometry. Each of these measuring processes has found its legitimacy in the industry, but no process has been able so far to convincingly cover the entire range, which is required to measure unknown surfaces.

With its patented working principle of structured illumination, Confovis provides a practical solution that removes the previously known constraints of classic confocal microscopy, in particular. This means that measuring results exhibit virtually no artefacts due to the patented technology, that even reflective surfaces can be measured optically and that roughness can be measured reliably, reproducibly and in a way that conforms to standards.

Precise measurement results with optical scanning process

Compared to the widely used tactile measuring devices, the Confovis system demonstrates its precision: with tactile scanning, the actual surface is filtered by the scanner radius, affecting the and the measured values recorded. With tactile scanning, the radius is at least 2 µm. By contrast, the Confovis system filters with a lateral resolution of up to 0.3 µm and thus resolves the surface structure more finely.