Optical measurement of surface topographies with transparent coatings

Manufacturers nowadays have access to state-of-the-art areal surface topography measurement instruments that allow investigation of surface topography at unprecedented levels of detail and over a wide range of scales. However, high value-added products have demanding requirements, pushing measurement technologies to their limits. Therefore, a deeper insight and more comprehensive understanding of performance and behaviour of current areal surface topography measurement solutions is often needed. In this work, we investigate and compare the results when measuring the same surface with different, state-of-the-art areal surface topography measurement solutions involving the principal optical technologies, notably focus variation microscopy, coherence scanning interferometry, imaging confocal microscopy and point autofocus instrument, operated using different set-ups. The test case is a highly engineered surface obtained through a sequence of mechanical and chemical surface modification processes. The surface has complex topographic formations at micrometre and sub-micrometre scales, and is characterised by the presence of a thin transparent layer, notoriously challenging for optical measurement. The topographies reconstructed from measurement are compared both in terms of visual appearance and texture parameters.