A novel, non-invasive, imaging methodology, based on the photoacoustic effect, is introduced in the context of artwork diagnostics with emphasis on the uncovering of hidden features such as underdrawings or original sketch lines in paintings. Photoacoustic microscopy, a rapidly growing imaging method widely employed in biomedical research, exploits the ultrasonic acoustic waves, generated by light from a pulsed or intensity modulated source interacting with a medium, to map the spatial distribution of absorbing components. Having over three orders of magnitude higher transmission through strongly scattering media, compared to light in the visible and near infrared, the photoacoustic signal offers substantially improved detection sensitivity and achieves excellent optical absorption contrast at high spatial resolution. Photoacoustic images, collected from miniature oil paintings on canvas, illuminated with a nanosecond pulsed Nd:YAG laser at 1064 nm on their reverse side, reveal clearly the presence of pencil sketch lines coated over by several paint layers, exceeding 0.5 mm in thickness. By adjusting the detection bandwidth of the optically induced ultrasonic waves, photoacoustic imaging can be used for looking into a broad variety of artefacts having diverse optical properties and geometrical profiles, such as manuscripts, glass objects, plastic modern art or even stone sculpture.

Due to climate change a slowly increasing annual temperature may be experienced by structures. Relative humidity (RH) fluctuations affect the equiibrium moisture content of materials. Repeated RH cycling leads to mechanical failure and may endanger an object's structural integrity. Preventive conservation is based on adopting measures that will prevent fracture. Real-time interferometry allows the acquisition of sequential high-resolution full-field surface images from hygroscopic materials used in cultural heritage by recording during cycles of changing RH. The differential images allow the development of a preventive methodology directly through surface responses. Indications of the natural onset of degradation can be followed and traced before visible damage occurs, allowing preventive measures to be taken in advance. An ongoing study (Climate for Culture European project (FP7-ENV-2008-1 CfC no. 226973)) aims to experimentally classify structural deterioration as a function of acclimatization and confirm the hypothesis that surface responses before deformation can indicate deformation threshold values as reference points for the onset of RH-induced deterioration.

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Cultural heritage conservation is an active field of research, where there is an ever‐growing demand for nondestructive and noninvasive diagnostic techniques, for performing remote analysis and diagnosis of the condition of historical structures and pieces of art, often of very high cultural and historical value. In this context, holographic interferometry is a very well‐established optical technique for research in cultural heritage, which brings together some very basic and critical properties such as contactless examination and nondestructivity, accuracy, repeatability, and a wide range of applicability. In this paper, the optical technique of digital holographic interferometry is tested on mock‐up, art‐related targets, with 2 different light sources, in an attempt to expand the technique towards a new approach that will profit from an easy‐to‐operate, inexpensive, and tunable source, offering a broad spectrum and wavelength selectivity, according to the needs of the experiments. Examples are presented, and the results demonstrate the effectiveness of the proposed modified experimental scheme for defect mapping, to be used in structural documentation reports, and for its exploitation in future hybrid optical diagnostic systems and data processing.

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This study proves the defect micro-morphology as a crucial parameter in conservation approaches. The study was performed on mural paintings and fresco samples which are considered as a major piece of European Cultural Heritage (CH). Nowadays the atmospheric pollution and various other extreme events provoke, more often than in the past, abrupt changes of environmental conditions, affecting negatively the murals state. Random contraction and expansion of the painting's complex structure is an important cause of extra load in its mechanical condition. This may eventually lead to the propagation of cracks within the layers, and expansion of the detachments. Furthermore, the sudden changes in the heterogeneous materials of the support can potentially trigger further damage to the multilayered painted surface. These subsurface damages may lead to unpredictable evolution of detachments and cracks, crucially affecting the painted surface. It is well known that a variety of techniques are employed to detect the location and the dimensions of the various detachments, hidden under the surface. In the present study emphasis is given on the importance of the detection and exact knowledge of the structural micro-morphology as a key factor to monitor the hidden, but ongoing, detachment processes. The precise monitoring of the detachments' micro-deformation will enhance the knowledge of the defect structure and help to visualize the interactions undergoing with the rest of the structure and thus will facilitate the choice of materials that will be used in the consolidation interventions, as well as their application guidelines. Hence, high resolution is of prior importance in monitoring the detection processes and as such laser metrology is implemented in this study.

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