Emerging photonic technologies for cultural heritage studies: the examples of non-linear optical microscopy and photoacoustic imaging

Abstract

The availability of non-invasive technologies, which can be used separately or in combination for obtaining chemical composition data and structural information of Cultural Heritage (CH) materials, is of prime importance for improving the understanding the environmental or ageing impact on monuments and artefacts and defining optimal strategies for their conservation. This paper overviews and assesses the potential of two emerging photonic technologies, the Non-linear Optical Microscopy (NLOM) and Photoacoustic (PA) imaging modalities, for a variety of diagnostic applications in preservation science. These techniques, which are well-established in biomedical research, during the last few years have been also investigated as non-invasive tools for the in-depth, high-resolution analysis of various CH objects, including paintings, documents and murals. We discuss on the applicability of these diagnostic optical methods to obtain precise stratigraphic information in artefacts, evaluating additionally the presence and the extent of potential morphological or chemical changes in several CH materials due to ageing. Furthermore, we demonstrate that the contrast complementarity of NLOM and PA imaging provides invaluable insights into the structural integrity of an artwork, which can be subsequently utilized for the early and accurate detection of depth degradation effects.

Current progress on murals: distribution, conservation and utilization

“…There are various methodologies for the on-site characterization of murals. Non-invasive techniques provide chemical information about the materials, and diagnostic methods can reveal structural information and provide multi-layered structures [ 73 ]. X-ray fluorescence spectrometer (XRF) [ 74 , 75 ], macro X-ray fluorescence (MA-XRF) [ 76 , 77 ], laser-induced breakdown spectroscopy (LIBS) [ 12 , 78 , 79 ], ultraviolet fluorescence [ 80 , 81 ], infrared [ 74 , 82 ], microwave imaging [ 83 – 85 ], Laser Raman Analyzer [ 13 , 50 , 86 ], and Terahertz (THz) spectroscope and imaging techniques [ 84 , 87 , 88 ] were widely used to study the materials of the paintings.…”

“…The non-invasive methods also have limitations as diagnostic tools in realistic cases since the materials of murals are numerous and heterogeneous [ 73 ]. The Raman, Ultraviolet fluorescence, and infrared can only be used to study the surface layers of the murals because none of them can penetrate thick layers of plaster.…”

“…In addition, X-rays and microwaves are suffered from no depth resolution and poor lateral spatial resolution [ 84 ]. Optical Coherence Tomography, digital holography, and Terahertz imaging can reveal the structure information and 3D mapping of the multi-layers of murals [ 73 ]. The ground penetrating radar can also be used to reveal the different layers of frescos and thus providing valuable information for 3D heritage/historic building information models [ 92 , 93 ].…”

“…It is worth noting that there are multiple machine learning methods, e.g., support vector machine, back propagation artificial neural network, K-nearest neighbor algorithm, random forest, and there are also other datasets such as laser-induced breakdown spectroscopy (LIBS) that could be used for the machine learning methods to identify the pigment compositions [ 78 ]. The detailed knowledge of chemical composition and physical structure may deepen our understanding of the history of the murals and thus can be helpful to evaluate their authenticities, as well as provide insights into the restoration and conservation of the murals [ 73 ]. Further studies including the spectra of pigments, plasters, and other artistic media are required to improve the applicability of this method for mural painting studies.…”