Revealing Hidden Features in Multilayered Artworks by Means of an Epi-Illumination Photoacoustic Imaging System

Abstract

Photoacoustic imaging is a novel, rapidly expanding technique, which has recently found several applications in artwork diagnostics, including the uncovering of hidden layers in paintings and multilayered documents, as well as the thickness measurement of optically turbid paint layers with high accuracy. However, thus far, all the presented photoacoustic-based imaging technologies dedicated to such measurements have been strictly limited to thin objects due to the detection of signals in transmission geometry. Unavoidably, this issue restricts seriously the applicability of the imaging method, hindering investigations over a wide range of cultural heritage objects with diverse geometrical and structural features. Here, we present an epi-illumination photoacoustic apparatus for diagnosis in heritage science, which integrates laser excitation and respective signal detection on one side, aiming to provide universal information in objects of arbitrary thickness and shape. To evaluate the capabilities of the developed system, we imaged thickly painted mock-ups, in an attempt to reveal hidden graphite layers covered by various optically turbid paints, and compared the measurements with standard near-infrared (NIR) imaging. The obtained results prove that photoacoustic signals reveal underlying sketches with up to 8 times improved contrast, thus paving the way for more relevant applications in the field.

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

“…As a consequence, these imaging systems are limited to the investigation of relatively thin objects, not exceeding a couple of millimeters in thickness, hindering the application of the method over a wide range of objects with diverse structural and morphological features. To overcome these restrictions, a novel optimized epi-illumination (reflection-mode) PA imaging system has been built, to provide universal diagnostic information in objects of arbitrary thickness or shape [55]. The capabilities of the epi-illumination PA apparatus were first demonstrated to reveal hidden charcoal, graphite or sinopia sketch drawings in thick concealed (gypsum, limewash, overpainted) wall painting mock-ups [56].…”

“…The promising capabilities of these imaging prototypes have been successfully utilized for the mapping of hidden sketches in painted mock-ups [59], as well as, the detection and study of restoration operations in an oil painting of historical significance [60]. In the latter case, it was observed that the area of the retouching was characterized by weak PA signals, due to the high acoustic attenuation properties of the restoration media, as well as, the back-reflections of [53][54][55] the main signals, resulting from the formation of various acoustic interfaces (Fig. 4d-e).…”

“…In this manner, while pure optical microscopy techniques operate in the ballistic photon propagation regime (up to 1 mm in scattering media such as soft tissues), PA diagnostic methods can extend the acquisition of diagnostic information to the optical diffusion regime, reaching otherwise inaccessible imaging depths which exceed 2-3 mm [30]. PA imaging has been directly compared to standard NIR optical imaging with regards to the mapping of hidden underdrawings in CH artefacts [53,55,59], demonstrating superior imaging performance in all cases. Furthermore, when compared to other competing diagnostic methods in CH including OCT, X-ray based techniques and THz imaging, PA modalities may offer higher penetration capabilities, improved imaging contrast, as well as, better cost efficiency, respectively [53].…”

Stratigraphy of Fresco Paintings: A New Approach with Photoacoustic and SORS Imaging

“…Photoacoustic imaging offers several applications in different scientific branches, ranging from biomedical to Cultural Heritage (CH) fields [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ], due to its capability to provide in-depth optical absorption contrast with high spatial resolution. In the field of conservation and artwork diagnostics, it has recently found several applications, such as revealing hidden underdrawings in paintings [ 8 , 9 ], discovering degradation and retouching features in historical oil paintings [ 6 ], and uncovering text in multilayered documents [ 10 ].…”

Computer Vision and Robotics for Cultural Heritage: Theory and Applications

“…Contrary to existing techniques considering light domes or free-form manual lighting, the article “LightBot: A multi-light position robotic acquisition system for adaptive capturing of Cultural Heritage surfaces” [ 2 ] uses a robot arm to set the light source at various positions with accuracy so that the system is adaptive to the size of the artefact and other constraints. However, to capture characteristics of paintings that are beyond its surface, the article “Revealing hidden features in multilayered artworks by means of an epi-illumination photoacoustic imaging system” [ 3 ] proposes a photoacoustic imaging system enabling the capture of hidden layers of paintings.…”