TESTING & MEASURMENT
New pioneering technology to objectively evaluate total perception of structured surfaces
Authors : Dr Christopher Groh , Gabriele Kigle-Böckler - BYK-Gardner GmbH
Abstract
Our visual perception is influenced by colour , gloss and the surface structure . Our visual rating takes all three parameters into consideration and makes an overall judgement . Up to now , grain or surface structure could only be judged visually or with high sophisticated microscopes . This has changed with the new spectro2profiler , a pioneering technology combining colour , gloss , 2D reflectivity and 3D topography in a robust , portable tool with a short measuring time .
Introduction - Colour analysis is not enough
Uniform appearance is a crucial quality criterion for many products . The choice of material and production process variations influence the surface quality , for example the cell size of powder coatings is highly dependent on paint film thickness and curing conditions . Another example are injection molding applications where fluctuations in tool pressure and temperature will show up in gloss and contrast variations . With the change of just one material or process parameter , the visual perception of colour and appearance can be changed significantly . So far visual assessment was the only way to deliver a complete judgement of a textured surface including colour and gloss evaluation . The recent developments of colour and gloss meters significantly improved quality control , but do not include the effects of structure or complex gloss behavior of 3D-surfaces . Therefore , 3D microscopes are used to provide very detailed information of the surface structure in the laboratory for research purposes , but not suitable for fast and easy analysis of production quality .
Figure 1 : Image acquisition of different illuminations to calculate surface topography ii
3D Topography analysis using photometric stereo technique
Photometric stereo is a technique for estimating surface normals in order to calculate a 3D topography of that surface . The technique was originally introduced by Woodham in 1980 . i The surface normals are calculated by observing an object from different illumination directions . With each direction , the object casts different shadows on the surface and a camera acquires images for each illumination ( Figure 1 ). Using shape from shading , the surface curvature is estimated , and the height map of the object can be calculated . The result is a real 3D topography of the measured object surface ( Figure 2 ).
52 JUNE 2021
Figure 2 : Height map of a powder coated surface measured with the spectro2profiler instrument . The unit P-μm is perceived height .
Watershed analysis to define structure cell sizes
Topographies such as leather grains or coarse powder coated structures can be characterized by their structure cells . To divide the topography into cells , the watershed algorithm is used , a regionbased segmentation approach . One can imagine that the algorithm
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