COMPANY SPOTLIGHT
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 gradually floods the valleys of the topography , building rivers until hill areas are surrounded . III These areas will be defined as cells , marked as green lines in Figure 3 .
Spatially resolved 2D reflectivity measurement
The spectro2profiler offers a new camera-based technology to capture the spatial distribution of reflectivity . . An in-line illumination setup eliminates cast shadows , invisible areas and perspective distortions so that the measurement is independent of orientation ( Figure 4 ). The camera acquires 2D reflectivity images . Figure 5 shows a grey scaled reflectivity map in which every pixel represents a reflectivity value allowing more detailed analysis of reflectivity distributions of a surface .
Figure 3 : Raw data and segmented topography data of a leather grain on a plastic substrate
Characteristic features of the surface can be calculated based on the watershed segmentation results to compare different structures or grains . Spatial length scales result from camera calibration and are traceable to SI-units . The calculated average cell size correlates to our visual impression of coarseness . The distribution of individual cell sizes is an indication for the uniformity of the surface structure . For example , a natural leather structure varies in uniformity depending on the part of the cow skin . A textured paint can form agglomerations during the wet paint application , if the application parameters vary resulting in an inhomogeneous appearance . The normalized cell size deviation is calculated by dividing the cell size distribution with the mean cell size . It is an objective measure to compare the uniformity of different structures independent of its absolute cell size .
Conventional measurement of reflectivity and gloss
Reflectivity and gloss are based on the interaction of light with the physical properties of the sample surface . The intensity is dependent on the material and the angle of illumination . The measurement results of a conventional glossmeter are related to the amount of reflected light from a black glass standard with a defined refractive index . Today ' s measuring instruments are very precise and widely used in industry , but they hold some weak points in the measurement of structured surfaces . Cast shadows and areas that are invisible to the measurement detector can falsify the measurement result . Moreover , the perception of gloss does not only depend on specular gloss but also on the observed contrast between specular highlights and diffusely reflecting surface areas IV . A conventional gloss meter is not capable to capture more complex reflective behaviour such as spatially distributed reflections e . g . high reflecting hills next to low reflecting valleys which occur in leather-like structures .
Figure 4 : Setup of spatial resolved reflectivity measurement
Figure 5 : Reflectivity map of a powder coated surface acquired with the spectro2profiler
CONTINUED ON PAGE 6 twitter : @ surfaceworldmag DECEMBER 2020 5