Combined photoacoustic imaging to delineate the internal structure of paintings.

In this Letter, we present a combined photoacoustic imaging method, based on consecutive excitation using either the fundamental or the second-harmonic wavelength of a pulsed Nd:YAG laser for the stratigraphy of painted artworks. Near-infrared excitation was employed for the imaging of hidden underdrawings in mock-up samples, whereas visible light was used for the thickness mapping of the overlying paint through the detection of photoacoustic signal attenuation. The proposed methodology was proven effective in measuring thick and strongly absorbing layers, which would not be possible by means of other pure optical techniques, while also enabling the visualization of features underneath the painted surface. Such an implementation expands significantly the applicability of the previously presented photoacoustic technique, which was limited to point-measurements, and paves the way for novel application in historical and technical studies, as well as in documenting restoring operations.