Nonlinear optical imaging techniques (NLO) for painting investigation

Nonlinear optical imaging techniques (NLO imaging), i.e. multiphoton excitation fluorescence (MPEF),second and third harmonic generation (SHG, THG), are high-resolution imaging modalities whichmay provide non-destructive determination of thickness and composition within multi-layer objectsas a function of depth. NLO techniques have been recently introduced into artworks analyses, giving promising results. As regards paintings, the guiding idea is to exploit the low absorption in thenear-infrared (NIR) of most materials, using a laser light in this spectral region which may generatenon-linear interactions within materials. The objective is to make feasible the in-depth profiling of thinfilms on the basis of refractive index changes, variation of optical activity and presence of fluorophores.Such information is definitely useful for the analysis of painted objects and can be also crucial for themonitoring of restoring operations. In this study, we present the preliminary results obtained throughthe application of NLO imaging techniques on a series of single- and multi-layers systems simulatingreal egg-tempera wood panel paintings. This research is part of the wider Iperion CH Project, whoseWP6 entails the evaluation of NLO-imaging to obtain highly resolved en-face and 3D images of smallareas on paintings. To that purpose, a set of multi-layer painted samples, simulating real paintings, wasdesigned and realized. They are currently under analysis with different operating systems and modalitieswithin the partnership. In order to assess the reliability of NLO measurements, data were complementedwith other well-established techniques, such as Fourier Transform Infrared Spectroscopy (FTIR)and X-Ray Fluorescence (XRF) for the chemical characterization of materials, Optical CoherenceTomography (OCT) for the determination of layers thickness and laser scanning micro-profilometryfor the study of surface morphology. In addition, colorimetric analyses, i.e. reflectance spectroscopy inthe visible region, were performed in order to chromatically characterize pigments and lakes. Finally,the transparency of the different materials was investigated by means of Scanning MultispectralVIS-NIR Reflectography.The preliminary results suggest that the 740 nm wavelength, while enabling the visualization of thepainting surfaces micromorphology, is not effective for the determination of thickness through thedetection of the non-linear signals.