## 3 Results and Discussion  
### Light-Induced Colour Changes in the Uncoated Wood Surfaces  
Light exposure of the uncoated wood surfaces (samples V1) resulted in progressive colour changes, visually perceived mostly as darkening, with some shade modifications, depending on the exposure situation and wood species. The evolution of these colour changes, expressed by the variation in the \(\Delta\)E, \(\Delta\)L*, \(\Delta\)a*and \(\Delta\)b* values as a function of the exposure time, is illustrated by the graphs in Figure 2, comparatively for the areas V1/Z3 directly exposed and the area V1/Z2 covered with a clear glass lamella. The total colour changes (\(\Delta\)E) in the unfinished wood support directly exposed to light/UV radiation (V1/Z3 areas) increased gradually during exposure, though these changes were more rapid in the first 24 h. The highest values after 72 h exposure were registered for maple (9.36 units), followed by ash (8.39 units) and then walnut (6.20 units). A similar order of light sensitivity for these three species was determined by [1], \(\Delta\)E values of approximately 14.1 units, 12.5 units and 9.8 units resulting after longer exposure (600 h). Colour changes of 12.5 units for maple and 11.7 units for walnut were reported after 7 years exposure to natural light passing through the window glass [48]. The total colour changes resulted mostly from the decrease in lightness, highlighted by the negative \(\Delta\)L* values evolving following a mirrored pattern when compared to \(\Delta\)E and reaching the final values of (-5.07) units for maple, (-4.13) units for ash and (-3.25) units for walnut (Figure 2(A2)). From a chromatic point of view, the positive \(\Delta\)a* values (up to 2.23 units) indicate only a slight increase in redness, while the positive \(\Delta\)b* values (up to 7.58 units) indicate a more important increase in the yellowness, specific to each wood species. The previous research and literature data indicated that the redness of wood, as well as UV/light-induced changes in redness, might be associated to the extractives [55], while the increase in yellowness can be correlated to UV-induced lignin degradation and formation of new quinones and stilbenes chromophores [8; 9; 11; 12]. As lignin photo-degradation is the first and main photochemical process induced by UV light, higher changes in yellowness compared to redness are expected. In the present research, the highest increase in redness (\(\Delta\)a*) was determined in the case of walnut (2.23 units), followed by maple (1.48 units) and ash with quite similar values (1.43 units). Contrarily, the lowest change in yellowness was determined for walnut (\(\Delta\)b* = 4.42 units), compared to ash (7.05 units) and maple (7.58 units), with the highest value. This is in accordance with the previous research results and may be explained by the role of extractives [23; 53]. Wood extractives may act as UV absorbers and free radical scavengers while themselves undergoing the photo-oxidation processes. All these effects result in retarding the photo-degradation of lignin in wood [18; 19; 20], which means less yellowing. A higher content of extractives in the walnut wood compared to maple and ash wood may explain both the darker natural colour of walnut and the reduced colour changes compared to maple following UV exposure, at least for the duration of the test carried out in this research (72 h). The light-induced colour changes for the uncoated wood surfaces that were covered with a 1 mm thick clear glass slide (V1/Z2) are presented in Figure 2B. It can be observed that the total colour changes (\(\Delta\)E) are smaller than those registered for surfaces directly exposed (V1/_Z3_) to UV light for all the three species, though the decrease in \(\Delta\)E values varied among species, being higher for maple and ash (approximately \(-4\) units) than for walnut (\(-1.24\) units). This shows an effect of the glass by diminishing the intensity/energy and very likely the spectral distribution of light reaching the wood surface underneath.