Speakers: Eleanna Pirgianaki (IESL-FORTH)

In recent years, photoacoustic (PA) techniques have gained significant role in the field of cultural heritage (CH) diagnostics, proving to be powerful tools for the detailed investigation of complex and fragile objects such as paintings, documents and murals. These techniques are based on the PA effect, which involves the generation of acoustic waves as a result of the absorption of intensity-variable optical radiation. The excitation and detection of PA signals through spectroscopic or imaging modalities is non-invasive and non-destructive and provides high optical absorption sensitivity at substantial depths within opaque media, surpassing the performance of several purely optical diagnostic methods.

In this context, we present a novel reflection-mode PA system integrating near-infrared pulsed excitation light and a spherically focused air-coupled ultrasonic transducer for the detection of the generated PA waves. The system is specifically developed to detect in-depth ageing effects in painted mock-ups and to reveal hidden underdrawings, such as pencil sketches, beneath non-transparent paint layers. To assess the performance of the system in detecting degradation phenomena, PA responses were recorded from different pigmented layers that had undergone controlled thermal or photo-ageing treatments. The measurements revealed a progressive increase in the PA amplitude over time, with distinct deterioration rates observed for various types of paint.

Additionally, by employing a second-harmonic excitation wavelength at 532 nm and comparing the PA signal ratios obtained from the two excitation lines, we aimed to enhance the method’s detection sensitivity while also expanding its applicability. Beyond the investigation of ageing effects in painted mock-ups, the developed dual-wavelength PA approach enabled the identification of corrosion levels in various metals and alloys, differentiation among inks of similar visual appearance, and detection of degradation effects in artificially aged papers. Overall, these results highlight the strong potential of PA detection as a complementary diagnostic technique to traditional methods, offering highly sensitive and non-destructive insights into degradation mechanisms and subsurface features across a broad range of CH artifacts.