This paper demonstrates how the full stratigraphy of a painted artwork can be non-invasively imaged by scanning the same area with multiphoton excitation fluorescence (MPEF) and photoacoustic (PA) imaging devices and combining the resulting cross-sectional images. This is particularly relevant within the heritage science field, for which the visualization of the stratigraphic structure of a painting is fundamental for understanding its original technique and state of preservation, and the non-invasive diagnostic is preferred to the invasive one. The proposed hybrid diagnostic approach applied on multi-layered painting mockups shows how MPEF depth-resolved images of the surface layers (varnish and paint layers) can be merged with the PA ones of the underlying layer (preparation with graphite underdrawings). Furthermore, by applying a novel convolution-based algorithm, the recorded MPEF signal has been decoupled into contributions arising from varnish and paint, allowing the determination of thickness and composition of the two individual layers. The highly complementary information provided by MPEF and PA techniques allows for the delineation of different stratigraphic layers in a painted artwork. The study lays the foundation for the creation of a compact imaging apparatus integrating multiphoton and PA contrast modes in a single instrument.
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