Nonlinear optical imaging techniques (NLO) for painting investigation

Abstract

Nonlinear optical imaging techniques (NLO imaging), i.e. multiphoton excitation fluorescence (MPEF), second and third harmonic generation (SHG, THG), are high-resolution imaging modalities which may provide non-destructive determination of thickness and composition within multi-layer objects as 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 the near-infrared (NIR) of most materials, using a laser light in this spectral region which may generate non-linear interactions within materials. The objective is to make feasible the in-depth profiling of thin films 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 the monitoring of restoring operations. In this study, we present the preliminary results obtained through the application of NLO imaging techniques on a series of single-and multi-layers systems simulating real egg-tempera wood panel paintings. This research is part of the wider Iperion CH Project, whose WP6 entails the evaluation of NLO-imaging to obtain highly resolved en-face and 3D images of small areas on paintings. To that purpose, a set of multi-layer painted samples, simulating real paintings, was designed and realized. They are currently under analysis with different operating systems and modalities within the partnership. In order to assess the reliability of NLO measurements, data were complemented with other well-established techniques, such as Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Fluorescence (XRF) for the chemical characterization of materials, Optical Coherence Tomography (OCT) for the determination of layers thickness and laser scanning micro-profilometry for the study of surface morphology. In addition, colorimetric analyses, i.e. reflectance spectroscopy in the 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 Multispectral VIS-NIR Reflectography. The preliminary results suggest that the 740 nm wavelength, while enabling the visualization of the painting surfaces micromorphology, is not effective for the determination of thickness through the detection of the non-linear signals.

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

“…The samples examined in this study are shown in Figure 1 (a detailed description of the samples can be found in [28]). Three pigments by ©Kremer (Germany) were selected for the paint layers due to their wide use in historical artworks, and also because of their specific optical properties.…”

“…In the last few years, pictorial layers were examined with the nonlinear modality of femtosecond pump-probe microscopy [26]. MPEF was also tested on paint layers [2] for the monitoring of the laser-removal of organic and inorganic acrylic paints [21] and for the evaluation of paint layer thickness with micrometric resolution [27,28]. As pointed out by these previous studies, the presence of highly scattering and/or absorbing media (pigments) in multi-layered painted systems tends to obstruct the detection of the NLOM signal, thus impeding the visualization of the stratigraphy.…”

“…As pointed out by these previous studies, the presence of highly scattering and/or absorbing media (pigments) in multi-layered painted systems tends to obstruct the detection of the NLOM signal, thus impeding the visualization of the stratigraphy. Specifically, MPEF imaging, performed with a laser wavelength set at 740 nm on the same set of samples analyzed in this study [28], enabled the visualization of the micromorphology of the painting surfaces but provided underestimated thickness values of the paint layers with respect to the real ones. Based on these results, it was hypothesized that a longer excitation wavelength could provide more reliable cross-sectional measurements.…”

Multianalytical non-invasive characterization of phthalocyanine acrylic paints through spectroscopic and non-linear optical techniques

“…In-depth monitoring of morphological and chemical degradation of varnishes was carried out through the combination of MPEF and THG modalities in the case of aging 28 or laser ablation 29 processes. Cross-sections of pictorial layers were obtained through the NLOM modality of femtosecond pump-probe microscopy [30,31] and, more recently, MPEF imaging has been applied for the determination of thickness of egg-tempera paint layers [32]. A recent work has illustrated the complementary capabilities of OCT and NLOM for the study of painting materials [25] and has warned on the possibility of damaging non-transparent layers by irradiating them with the excessively high fluences of the femtosecond used in non-linear microscopies.…”

“…A recent work has illustrated the complementary capabilities of OCT and NLOM for the study of painting materials [25] and has warned on the possibility of damaging non-transparent layers by irradiating them with the excessively high fluences of the femtosecond used in non-linear microscopies. Dal Fovo et al [32] have examined tempera paint layers by MPEF upon femtosecond excitation at 740 nm and have showed that their thickness was underestimated due to the strong absorption and attenuation of the emitted fluorescence. Based on these antecedents, the main limitation of NLOM imaging seems to be associated with the presence of highly scattering and/or absorbing media (pigments), which obstruct the detection of the NLOM signal, and of the high laser fluences needed to excite a measurable non-linear optical signal.…”