## 3 Results  
### Multispectral Imaging and and Paint Layer Thickness Measurement  
UV and IR imaging of the original banknote network by RichardHTT (Table 1) revealed distinct responses from the applied materials, enabling the identification of unpaired areas, integrated security features, and underlying preparatory sketches. Under UV illumination, the hair of the female figure, appearing blue and lilac under visible light, showed differentiated behaviour: the lilac portions emitted a reddish fluorescence, while the blue areas appeared dark, indicating a lack of fluorescence. The red background demonstrated strong UV absorption, as did other colours such as the blue of the hair and the yellow of the necklace beads, all of which lacked notable fluorescence. Infrared reflectography revealed the presence of underlying sketches and preparatory lines, offering insights into the artwork's execution process. Notably, two specific zones, the soldier's shoulder and the woman's hand, exhibited distinct UV fluorescence patterns, revealing unpainted areas or possible use of a white pigment with different optical properties. These anomalies may suggest either incomplete coverage by the POSCA markers or the intentional use of a fluorescent pigment differing from the rest of the palette. The analysis of the artist's original banknote provided valuable insights into the optical behaviour and material characteristics in a real artistic context (Table 2). In parallel, the experimental banknote was designed to replicate these conditions, allowing for a systematic evaluation of the effects of artificial ageing and protective treatments under controlled laboratory conditions. Before the application of protective coatings, the chromatic fields of the experimental banknote exhibited notable responses under different lighting conditions. Specifically, the pink area displayed an intense and distinctive red fluorescence under UV light, while in infrared reflectography it showed high IR reflectance. The remaining colour fields did not fluoresce under UV and appeared dark in both UVF and UVF-Y images. In infrared reflectography, the black field appeared transparent to IR radiation, clearly revealing the banknote substrate. The white, light blue, yellow, and red areas appeared brighter than the substrate, indicating strong IR reflection. \begin{table}
\begin{tabular}{c c c} \hline
**ID** & **VIS** & **UVF-Y** & **IR960** \\ \hline Artwork & & \\ \hline \end{tabular}
\end{table} 
Table 1: Multispectral imaging investigation on the artist’s banknote. Images were acquired using three acquisition modes: visible light (VIS), ultraviolet fluorescence with a yellow filter (UVF-Y), and infrared reflectography with a long-pass filter at 960 nm (IR960). aged areas made inscriptions and markings on the banknote more visible, due to decreased IR reflectance within the pigment layer. Following the application of the protective coatings, no significant differences were observed in UV fluorescence responses between treated and untreated areas, either before or after ageing. In infrared reflectography, however, the aged and coated areas showed greater IR absorption compared to the corresponding untreated areas, suggesting that ageing altered the optical properties of the coating materials. Thickness measurements (Table 3) performed on both the original artwork and the laboratory samples confirmed that the pictorial layer applied by the artist [PERSON] (120 \(\upmu\)m \(\pm\) 62 \(\upmu\)m) closely matches the values obtained in the laboratory reproductions using POSCA markers. The coloured paint layers, before the application of any protective treatment, showed an average thickness ranging from approximately 90 \(\upmu\)m to 140 \(\upmu\)m, depending on pigment colour and application density.