A study of potential laser-induced degradation in remote standoff Raman spectroscopy for wall paintings

Abstract

A mobile remote standoff Raman spectroscopy system operational at typical distances of 10 m was developed specifically for research of historical sites and wall paintings recently. Here we present an upgrade to that system informed by a thorough experimental investigation of the relevant laser-induced degradation issues. Reflectance spectroscopy as a more sensitive technique than Raman spectroscopy was used for monitoring and a new phenomenon of reversible alterations was detected in many paint samples at very low laser intensities of less than 1 W/cm2 when Raman measurements detected no changes. Contrary to conventional wisdom, the intensity threshold for safe operation was found to decrease significantly for larger incident irradiation area in the case of a vermilion oil paint sample. Damage threshold in intensity for each material needs to be determined for different spot sizes, which can be orders of magnitude lower for 1 mm spot size compared with micro-Raman. Results from this study is also relevant to portable Raman systems which use similarly large spot sizes. However, the larger spot size still generates more Raman photons overall under safe operation than micro-Raman systems. Continuous-wave (CW) lasers are found to be best suited to efficient, that is more Raman signal detected over a given measurement time, and safe Raman operation than ns-pulse lasers at the same wavelength. While the damage threshold in intensity for ns-pulse lasers is much higher than that of CW lasers, the pulse energy allowed in one pulse for safe operation is still too low to allow detection of Raman signal, and the need for multiple pulses makes pulse laser inefficient owing to the low repetition rate necessary to ensure adequate heat dissipation between pulses. The safety of the upgraded system was evaluated and found that no permanent laser-induced degradation was detected within 60 s of laser irradiation for any of the paint samples.

Advanced procedures in Raman forensic, natural, and cultural heritage studies: Mobile set‐up, optics, and data treatment—State of the art and perspectives

“…This technique starts also to be used for cultural Heritage studies. 9,[89][90][91] The two sets of spectra were visually similar, supporting the potential use of a handheld Raman spectrometer with stand-off attachment for blood detection at the scene of crime.…”

“…Finally, Almehmadi and Lednev 79 collected bloodstain spectra using a handheld Raman spectrometer equipped with a standoff attachment and compared with spectra acquired with a benchtop Raman microscope. This technique starts also to be used for cultural Heritage studies 9,89–91 . The two sets of spectra were visually similar, supporting the potential use of a handheld Raman spectrometer with stand‐off attachment for blood detection at the scene of crime.…”

“…Its usefulness is easily understood for forensic applications (e.g., when the crime scene cannot be contaminated) 79 but also for cultural heritage cases (e.g., for wall paintings and architectural interiors). [89][90][91] The paper by Rui-z-Moreno et al, 86 instead, investigated the potentiality of using different spectral ranges to exploit the information deriving from photoluminescence (PL). Actually, various research groups have recently made an important effort to try and make the best possible use of the spectral information often considered as "simple background."…”

“…The optics problem of focusing and collecting scattered light from spots that cannot be reached by the operator, who, for various reasons, needs to stay far away from the analyzed material, can be overcome by the employment of stand‐off Raman spectroscopy. Its usefulness is easily understood for forensic applications (e.g., when the crime scene cannot be contaminated) 79 but also for cultural heritage cases (e.g., for wall paintings and architectural interiors) 89–91 …”

Time and spatially resolved VIS-NIR hyperspectral imaging as a novel monitoring tool for laser-based spectroscopy to mitigate radiation damage on paintings

“…16 Our recent studies found that the two commonly employed inspection techniques, visual observation and Raman monitoring, are also not sensitive enough to detect some short-term alterations on paints that can be detected only with visible and near infrared (VIS-NIR) reflectance spectroscopy. 6,17,18 Although visual inspection and Raman spectroscopy can offer useful guidance for the detection of certain laser-induced damages, they can be blind to physical changes (e.g., partial melting of the surface) and inter and intra molecular scale damage below the detection limit. In some cases, the alteration may not correspond to damage of the pigment (e.g., alteration in the binding medium).…”