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Optical Coherence Tomography (OCT) has proven to be an extremely valuable tool for the non-invasive stratigraphic analysis of paintings. One of the important components of OCT is the broadband laser source. The recent availability of off-the-shelf white light supercontinuum laser sources has meant that OCT systems can be built at any wavelength in the visible and short-wave infrared (SWIR) regime (~400-2500 nm). The most common OCT systems are those built for biomedical applications and they tend to be centered around near infrared spectral regions with low water absorption coefficient, such as 800nm and 1300nm. Historic artist materials such as paints tend to be highly scattering which limits the OCT depth of penetration at these wavelengths. A systematic study was conducted on paint layers composed of common artist pigments in egg tempera and oil to explore the optimum spectral window for maximum depth of penetration for OCTs and found it to be around 2.2 µm over the VIS/SWIR range. An OCT system was then developed at 1960nm and demonstrated to provide superior depth of penetration compared with lower wavelength OCT systems for highly scattering paint and other materials. Recently, the development of broadband laser sources has been extended to the mid-IR region. Consequently, there has been a growing interest in developing OCT with ever longer wavelength, beyond the SWIR, in the hope to increase the depth of penetration even further. To explore this in the mid-IR spectral region, we collected the FTIR spectra of a large selection of paint materials prepared as paint outs in external reflection and transmission mode in the region 4000 - 400 cm-1 (2.5 - 25 µm). With this work, we aim to systematically explore whether by extending the wavelength of OCTs to the mid-IR region would allow deeper penetration into objects of interest for cultural heritage science.
To explore this in the mid-IR spectral region, we collected the FTIR spectra of a selection of paint
materials prepared as paint outs in external reflection, transreflection and transmission mode in the region 4000-400
cm-1 (2.5-25 µm). External reflection (ER)-FTIR spectroscopy is a suitable tool for the non-invasive
chemical characterization of materials and presents significant advantages (no need of sampling and
detection of combination/overtone bands) compared to the other collection modes [4]. However, its use
is still limited as FTIR spectra collected in ER mode, particularly in specular geometry, are much more
challenging to interpret compared to FTIR spectra collected in Attenuated Total Reflection (ATR) and
transmission mode. This is due to strong distortions affecting fundamental IR bands of ER-FTIR
spectra, which are a direct consequence of the reflection phenomenon for absorbing materials in mid-IR. Multiple factors affect the resulting ER-FTIR spectra, including the material refractive index,
absorption coefficient, roughness of the surface and particle size. Hence, the same pigments in egg
tempera and oil applied on different substrates as paint outs were measured.
Finally, the FTIR spectra collected will be used to compile a spectral database of paint materials, which
will contribute to the advancement of ER-FTIR spectroscopy for non-invasive chemical analysis of
materials.
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