*2.2. Vis-NIR Multispectral Reflectography*

Multispectral analysis was performed with the scanner developed at the National Research Council—National Institute of Optics (CNR-INO), allowing for the simultaneous acquisition of 32 narrow-band images (16 VIS + 16 NIR images) through whiskbroom scanning in the range 390–2500 nm [5]. The optical head, i.e., the lighting system and catoptric collecting optics, is placed in a 45◦/0◦ illumination/observation geometry, moving with a step of 250 μm and speed of 500 mm/s. The light reflected from the scanned surface is collected by a square-shaped fiber bundle and delivered to a set of Si and InGaAs photodiodes, each equipped with an individual interferential filter. The autofocus system maintains the optimal target-lens distance during scanning thanks to a high-speed triangulation distance meter and custom-made control software. The instrument's output is a set of perfectly superimposed monochromatic images, which are aberration-free and metrically correct.

The multi-spectral image cube was processed with principal component analysis (PCA) to compress the informative content of large amounts of data in a new, reduced, non-redundant dataset [29]. PCA allows for the expression of the original spectral dataset

within a new reference space identified by orthogonal and uncorrelated coordinates, called principal components (PCs), corresponding to linear combinations of the original variables (i.e., the different wavelengths). Since PC variables are hierarchically ordered, the few first PC images are typically representative of the substantial information of the original dataset [1]. Given our research aim, the initial 32 images were reduced to four significant PC images that effectively summarized the salient spectral variations of the drawing. Vis, NIR, and PC images were used to produce false-color (FC) images by combining either a near-infrared, a red, and a green image (NRG-FC), or three PCs (PC-FC). In the first case, traditional NRG→RGB mapping was used (namely, N→R, R→G, and G→B, with "R", "G", and "B" that indicate the red, the green, and the blue channel, respectively, and "N" the near-infrared spectral band). In the second case, three PCs were combined in the trichromatic RGB space. The resulting image provided a detailed outline of the drawings and their similarities—when present, which would otherwise remain undetected during a simple visual inspection, and to a traditional FC.

## *2.3. Laser Scanning Microprofilometry*

Morphological analysis was carried out using a laser scanning micro-profilometer developed at CNR-INO for the survey of a wide range of materials and surfaces. A commercial conoprobe (Conoprobe 1000, Optimet, Jerusalem, Israel), equipped with a 50 mm lens, is moved by a scanning device allowing for measurements on a maximum area of 30 × 30 cm2. The profilometer has 1 <sup>μ</sup>m axial resolution, 20 <sup>μ</sup>m lateral resolution, and 8 mm dynamic range. The output is a faithful, high-resolution topographic map of the measured surface, which may be displayed either as a 3D model or as an image. The latter may be further processed through the application of digital filters and rendering techniques to enhance micrometric details and improve their readability.
