Uncovering the hidden content of layered documents by means of photoacoustic imaging

We introduce a novel diagnostic approach for the uncovering of hidden content in layered documents, based on a photoacoustic imaging setup with optimized acoustic detection parameters. By exploiting the laser-induced ultrasound following the intense absorption of pulsed optical radiation by printed ink, this traditional biomedical imaging method is proven capable of revealing text characters buried under several layers of stacked paper sheets at high spatial resolution and excellent contrast levels. Such a remarkable performance benefits from the effective combination of strong light scattering by paper fibrils and the virtually unobstructed propagation of the generated ultrasonic waves through multiple layers, enabling thus the accurate recording of text, even in double-sided prints, with minimum shadowing artifacts. The imaging effectiveness, the implementation simplicity, the robustness and the relatively low cost features offered by the proposed photoacoustic modality, are anticipated to contribute in its wide adoption by the preservation of cultural heritage community, as a powerful diagnostic tool for the in-depth investigation of complex multi-layered objects.