Multiscale roughness analysis by microprofilometry based on conoscopic holography: a new tool for treatment monitoring in highly reflective metal artworks

Abstract

The analysis of surface roughness in highly reflective metal artworks is challenging and requires contactless devices capable to measure regions with high micrometer accuracy in both depth and lateral directions. We demonstrate optical profilometry based on scanning conoscopic holography for micrometer measurement of silver samples treated with different hand-made cleaning processes. The technique is shown effective in acquiring shiny and smooth metal samples providing high-resolution and high-accurate dataset ($$0.1\,\upmu \hbox {m}$$ 0.1 μ m depth and $$5\,\upmu \hbox {m}$$ 5 μ m lateral resolution) that is a reliable representation of the microsurface structure. From a statistical point of view, the cleaning treatments have the same nature of the low-abrasion, but the underlying mechanical processes are different. This fact suggested a more in-depth study of both the amplitude and the hybrid areal roughness parameters. It is proposed a workflow for a dual integrated multiscale roughness analysis for surface characterization: a scale inspection to detect possible texture non-homogeneity, and a signals separation to outline the most significant texture components. The scale-limited components allowed to discriminate the different surface processes. The results on silver samples demonstrate the potential of multiscale roughness analysis by conoscopic holography as a new tool for treatment monitoring in metal artworks.

Can we unlock more information from interferometric sensors? Feasibility and performance analysis

“…A detailed description of the prototype can be found in the literature. 9,10,12 The hardware and the data modules of the microprofilometer is exemplified in Figure 1. The holographic probe is based on a laser diode of 655 nm wavelength that allows single-point distance measurements in the line-of-sight direction.…”

“…6 Our research efforts have resulted in the development of a prototype with a versatile setup suitable for out-oflaboratory measurements 7 based on multiple conoscopic holography sensors and a multi-step data pipeline that provides high-quality surface dataset on heterogeneous (diffusive, highly reflective, or polychrome) artworks. The technique was demonstrated in monitoring microclimate decay in paper and wooden paintings, 8 in a multiscale roughness analysis workflow used to endorse treatments in silver artworks in collaboration with conservators, 9 and in the in-process monitoring of a ancient painting surface based on metrology based on ISO standard descriptors. 10 From the work done, it has been learned that a fundamental requirement for accurate analysis of surface topography is spatial registration because of the lack of references in the height data with respect to the "visually readable" surface.…”

Optical surface metrology for heritage science: proof of concept and critical-constructive discussion

“…PoC of multiscale analysis for monitoring silver treatments was given by the authors in Ref. 10. The workflow is depicted in Fig.…”

“…In heritage science literature a number of optical techniques are reported for noninvasive acquisition of the artwork surface microtexture, from morphology inspection to in-process metrology. [6][7][8][9][10] Among common 3D profilometry techniques are laser triangulation, 11 structured light projection, 12 focus variation microscopy, 13 white light interferometry. 14 In this work, surface data are obtained by laser profilometry based on conoscopic holography interferometric sensors.…”

“…15 This technique is well accepted in the heritage network, allowing versatile applications from archaeology 16 to paintings, 9,17 and more recently to book heritage. 18 We employed a scanning profilometer that we had customized for wide-field and on-field diagnostics of artworks, 10,19 able to provide an accurate dataset on heterogeneous (diffusive, highly reflective, polychrome) artworks. This paper starts from a review of our recent research in surface metrology, 10,17,18 showing the Proof-of-Concept (PoC) of some applications targeting exemplary problems, with the aim of discussing a road-map towards an effective use of surface profilometry on artworks.…”