Stratigraphic mapping of paintings by multispectral reflectography

Despite recent technological advances, it is currently not possible to uniquely and noninvasively measure the micrometric thickness of pictorial layers in paintings. The presence of optically opaque materials (pigments) severely limits the detection capability, as it hinders the penetration of the probe's near-infrared radiation, typically employed for this purpose. In our previous work, we explored the possibility of using diffuse reflectance spectroscopy (DRS) to achieve stratigraphic information about painting materials. We showed that the thickness of pictorial layers can be quantified by the intensity of the spectral reflectance factor measured at a given wavelength in the infrared, i.e., an easily detectable spectral feature. In the present work, we extend the previously proposed method by applying multispectral reflectography to measure new ad hoc prepared pictorial samples and a mock-up simulating a Mondrian painting. We thus demonstrate that it is possible to obtain 2D and 3D stratigraphic maps of the pictorial layer over large areas with the same measurement time and spatial resolution as multispectral scanning reflectography.