engleski

Suitability of optical topography measurement methods for the assessment of graphic papers

The topography of paper has a great influence on the print result. Therefore the surface structure of paper, roughness or complementary smoothness, is an important parameter for the quality control in the paper mills. Numerous devices based on various principles have been developed and brought to market for smoothness measurement, whereby the most common are indirect methods based on the air leak principle. These devices however reach their limits when it comes to differentiation of very smooth papers which are nowadays common on the market. Furthermore the porosity of paper influences the air leak measurements as well and the real topography stays inaccessible. Nonetheless these devices are very common because there are easy to operate. Furthermore the interpretation of results is simple, due to the fact that the database for paper is large enough, so that every paper can be assigned to a different roughness category. The direct smoothness measurement, such as profilometry is also common in the paper industry. Direct topography evaluations are based on both contacting (stylus-type instruments) and non-contacting (optical) methods, with optical systems being predominant in the paper industry because they cause no damage to delicate paper surfaces. Measuring principles used for optical evaluations include white light interferometry, laser auto-focussing, phase-measuring stripe projection, focus variation, confocal microscopy and others. In this work the principles of phase-measuring stripe projection and focus variation were examined and their applicability for the evaluation of paper topography determined. The main focus of the work was to define the parameters necessary to generate a paper topography image, such as necessary vertical and lateral resolution, taking the printing process parameters and paper structures into account. Further issues were the handling of samples and how analytical results are influenced by the image acquisition procedures of the two principles. The topography images are commonly used for calculation of roughness parameters, whereby the corresponding standards originate form the metal industry (DIN EN ISO 4287) and the suitability of their application on the surface of paper has been examined. Especially the issue of topography image filtering for the derivation of roughness parameters was analysed and appropriate parameters defined. The topography parameters obtained by these optical methods were verified by quantitative evaluations of high-resolution SEM images of ion-etched cross-sections.

Topography; Roughness; Surface; Surface Roughness Parameters

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