## 1 Introduction  
Colour, alongside texture and grain, represent particular and valuable aesthetic features of the various wood species [1]. The coating of wood surfaces with transparent, colourless finishing materials is the usual treatment applied to highlight and preserve their beauty and natural aspect over time in various practical applications. Nevertheless, colour changes in both uncoated and coated wood surfaces occur over time when exposed to light/UV radiation, both indoors and outdoors. This practical issue has been only partly solved by the outcomes of extensive research worldwide, remaining an actual research topic [2; 3; 4; 5; 6; 7]. Colour changes in wood surfaces exposed to light are primarily the result of a complex and dynamic photo-degradation process of the wood surface, having as main actors the UV radiation from the light source and the lignin in wood. The absorption of UV radiation by lignin results in chemical bond cleavage and the formation of reactive free radicals (e.g., phenoxyl), which are then transformed into carbonyl- and carboxyl-containing chromophores and quinoid structures, responsible for colour changes, especially yellowing of the exposed wood surfaces [8; 9; 10; 11; 12; 13]. Not only UV light but also visible violet light and blue light up to 515 nm may cause surface and subsurface colour changes without affecting lignin [14; 15]. Furthermore, a possible contribution of wavelengths in the visible-near infrared region was highlighted by hyperspectral imaging [16]. In addition to lignin, certain classes of wood extractives may play an important role in determining the natural colour of wood species and their behaviour when exposed to light. Poly-phenolic extractives, such as tannins, can absorb UV light, scavene free radicals and undergo photo-oxidation, retarding the photo-degradation of lignin and wood [17; 18; 19; 20]. On the other hand, extractives from dark coloured wood species are responsible for their sensitivity to visible light, resulting in a bleaching effect after longer exposure times [21]. The result is, therefore, that the light-induced colour changes in wood surfaces are significantly influenced by the wood species due to the differences in their chemical composition, as proven by comparative research on more than two different species [1; 22; 23; 24]. Colour changes also depend on the type of light (natural/artificial), spectral range (wavelengths), type of light source, exposure conditions and duration [25; 26; 27]. Accordingly, data from different research publications are often difficult to compare. The coating of wood can impart some protective effect against light-induced colour changes by reducing the amount of light/UV radiation reaching the underlying wood substrate [28], depending on the chemical structure of the film-forming resin and the specific additives in a certain formulation. The coating materials can also undergo photo-degradation processes (photolysis, photo-oxidation) under the action of UV light, resulting in property changes, including glass transition temperature, UV light transmittance, colour and their protective behaviour [29; 30]. Different types of film-forming materials, such as: acrylic-polyurethanes [31], acrylics [28; 29; 32; 33; 34] and polyurethane [35], as well as natural products [36], have been studied in terms of behaviour and protective effect against light/UV-induced colour changes in wood surfaces. Due to their transparency for both visible and UV light, clear finishes offer generally less protection against the action of light/UV radiation, resulting in more pronounced colour changes [28; 29; 31; 33; 35], compared to semi-transparent and opaque finishes that contain colouring pigments with a UV protective effect. The interaction of UV light with the underlying wood at the interface wood/coating material results in the photo-oxidation of lignin and associated colour changes, representing an important contribution to the overall colour changes in clear-coated wood surfaces [10; 28; 35].