## 3 Results and Discussion
### Light-Induced Colour Changes in the Coated Wood Surfaces
The pattern for ash was also different, with a less pronounced decrease in yellowness in the first 24 h, but finally after 72 h, there were no significant changes in yellowness, similarly to the uncovered samples. For walnut, the evolution pattern did not change shape, but the final increase in yellowness was slightly increased. Exposure through the covering glass reduced the variation in lightness to different extents depending on the wood species but did not change visibly the evolution patterns. It can be noted a smaller lightening for walnut, while the darkening effect for maple and ash seems less affected. Accordingly, there were some small differences in the evolution patterns of \(\Delta\)E values for the three species and the final values recorded after 72 h light exposure. Based on these values, the colour stability of the wood samples coated with the waterborne polyurethane (1 K) lacquer F3 and covered by a 1 mm thick clear glass slide (S) decreased in the following order: S-F3-ash \(\geq\) S-F3-walnut > S-F3-maple, which is slightly different from those observed earlier for direct exposure (F3-ash > F3-walnut \(\cong\) F3-maple), especially as walnut is concerned. It has to be pointed out that for this type of finish (F3), the glass cover showed sort of a shielding effect only in the case of walnut, reducing the final \(\Delta\)E value by 1.41 units, whilst the corresponding values for maple and ash substrates actually increased with 0.16, respectively, 0.35 units. More research on an increased number of replicates and complementary investigation methods, including spectral analysis of the light emitted by the source and passing through the 1 mm thick clear glass cover, as well as the interaction of the respective light with the different wood substrates and the coating films in terms of absorption, reflection or transmittance are necessary to better understand and explain the experimental results obtained so far. Light-Induced Colour Changes in the Wood Substrate under the Coating Film, Simulated by V1/Z1 Zones--Contribution of the Underlying Wood Substrate to Overall Colour Changes  
The experimental method employed in this research envisaged a possibility to highlight, measure and compare the light-induced colour changes in the wood substrate under the coating film and in the coating film itself. For this purpose, sort of a \"detachable\" coating film was necessary, and this goal was achieved by applying the coating films on 1 mm thick clear glass slides. These were tightly attached to the uncoated wood substrate during light exposure and removed after each exposure period to allow evaluation of the colour changes in the coating film (on glass slide) and in the substrate under the protective coating film (areas V1/Z1). As was already mentioned, the influence of the 1 mm thick clear glass slide was a limitation of this method and was considered by comparative evaluations with areas V1/Z2 (exposure through clear glass). The graphs plotted in (Figure 6) illustrate the in-time evolution of the global colour changes, expressed as \(\Delta\)E values, for wood substrates exposed to light under the three different types of coating films: acrylic-polyurethane (F1), acrylic (F2), polyurethane (F3) applied on glass slides (SF1, SF2, SF3), alongside the control curves highlighting the effect of the clear glass (S). It can be clearly observed that light penetrated through the transparent coating film and the clear glass slide, reaching the wood substrate underneath and causing colour changes that evolved with the duration of exposure. The evolution patterns in the colour changes were very similar for exposure under clear glass (S) and exposure under coated glass (SF1-SF3), with only some small differences in the actual \(\Delta\)E values. Slightly lower values under the coated glass may suggest some protective effect of the respective coating films, whilst slightly higher values may indicate an opposite effect.