On the use of a zoom lens for remote Raman measurements: Application in cultural heritage research

Raman spectroscopy is frequently implemented in the analysis of art objects. Especially its non‐destructive character and the advent of mobile instruments, often equipped with fibre optics probe heads, have opened the doors for many direct investigations of artworks. However, Raman spectroscopy usually requires stable positioning of the objective lens, close to the paint surface. Consequently, and despite the use of an extensive range of positioning equipment, it is not possible to reach every spot of a large artefact. Positioning, and as a consequence measuring, is becoming even more critical for 3D objects. Therefore, we examine the characteristics and explore the opportunities for the investigation of art objects that can be obtained by using a zoom lens, which is suitable for the Raman investigation at working distances between 6 and 60 cm. It is shown how a zoom objective can be characterised and used for remote Raman investigations at working distances ranging between 6 and 60 cm. The feasibility of this approach is illustrated by the analysis of two art objects: a polychrome sculpture from the Ducal Palace of Vila Viçosa (Pt) and hand‐painted Chinese wall papers from the collection of the Museu Nacional de Arte Antiga (Pt).