## 2 Materials and Methods
### Colour Measurements  
Colour measurements in the CIELab reference system were performed using an Avaspec-USB2 spectrometer from Avantes (Apeldoorn, The Netherlands) [51] equipped with an integration Ava Sphere with a diameter of 80 mm, with a circular measuring aperture of 8 mm in diameter. Measurements were carried out under a standard D65 illuminant and the measurement angle set at 10 degrees, according to the relevant standard methods [52]. The light connection to the sphere was via glass fibres and a COL-UV-VIS collimating lens connected to the light source. The colour data were processed employing Ava soft 7.0. The measured colour parameters were: the lightness value (L*), varying from 0 for black to 100 for white; the degree of red (redness) on the green-red chromatic axis (\(-\)a*, \(+\)a*); and the degree of yellow (yellowness) on the blue-yellow colour axis (\(-\)b*, \(+\)b*). The colour measurements were performed on 12 fixed points (circular areas of 8 mm diameter) on each wood sample (uncoated or coated) and on 3 fixed points for the coated glass lamellae, \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Wood Species Substrate**} & \multirow{2}{*}{**Experimental/Calculated Value**} & \multicolumn{3}{c}{**Type of Coating**} \\ \cline{3-5}  & & **F1** & **F2** & **F3** \\ \hline \multirow{2}{*}{Maple} & Application rate [g/m\({}^{2}\)] & 191.1 \(\pm\) 16.2 & 196.3 \(\pm\) 19.7 & 203.5 \(\pm\) 20.2 \\ \cline{2-5}  & Film thickness [\(\upmu\)m] & 47.8 & 57.9 & 55.6 \\ \hline \multirow{2}{*}{Ash} & Application rate [g/m\({}^{2}\)] & 180.5 \(\pm\) 25.8 & 203.3 \(\pm\) 17.4 & 168.3 \(\pm\) 4.0 \\ \cline{2-5}  & Film thickness [\(\upmu\)m] & 45.1 & 59.9 & 46.0 \\ \hline \multirow{2}{*}{Walnut} & Application rate [g/m\({}^{2}\)] & 161.6 \(\pm\) 16.2 & 184.9 \(\pm\) 28.6 & 189.4 \(\pm\) 17.4 \\ \cline{2-5}  & Film thickness [\(\upmu\)m] & 40.4 & 54.5 & 51.8 \\ \hline Glass lamella & Film thickness [mm] & 0.034 \(\pm\) 0.004 & 0.043 \(\pm\) 0.008 & 0.046 \(\pm\) 0.007 \\ \hline \multirow{2}{*}{Coating material} & Solids content [\%] & 25.0 & 29.5 & 27.3 \\ \cline{2-5}  & Flow time, \(\phi\) 6 mm, 20 \({}^{\circ}\)C [s] & 12 & 49 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental values referring to the preparation of the coated wood samples and glass lamellae with the three types of coating materials: application rates, theoretical film thickness on wood samples, thickness film on glass lamellae, alongside the solids content and viscosity of the applied coating materials F1, F2, F3. Figure 1: Test samples: (**A**). V1—Uncoated wood support, coating on glass lamella: dimensions and various specific zones/light exposure situations (Z1–Z4); (**B**). V2—Coated wood support: dimensions and various specific zones/light exposure situations (Z1–Z3). (reproduced from [46] with permission). prior to assembling the wood specimens with the glass lamellae. After each period of light exposure, the samples were disassembled and colour measurements repeated in the same areas on the wood substrate and the coated lamellae. The colour measuring points on the wood samples corresponded to 3 fixed locations on each zone of the test samples, excepting zones V2/Z1 with 6 measuring points. As the test samples were in triplicate, a minimum number of 9 and maximum 18 measurements resulted for each zone/exposure situation for each tested variant.