## 4 Discussion
It was previously reported that the light-cured resin cement Variolink II (Ivoclar Vivadent), also includes ytterbium trifluoride, and improved the fluorescence level of the e.max Press ceramic restoration [28]. On the other hand, the manufacturer of RelyX Veener cement does not report luminophores in its composition. This may possibly explain why the fluorescence level was lower than in the other groups. In the groups with ceramic thickness of 0.8 mm, a greater influence of the shade of the cement was appreciated between cements of the same brand. Darker shades within the same brand obtained a lower emission of the final fluorescence of the restoration. Thus, for example, veneers cemented with RelyX Veneer A3 had lower fluorescence values than RelyX Veneer Translucent. The same was observed with G-aenial Flo A3 compared to A2, and Variolink Veneer Warm compared to Light and Neutral. This trend was also confirmed in a recent paper in which the authors investigated the fluorescence behavior of different shades of selected contemporary tooth-colored restorative materials and concluded that--within any one brand of material--fluorescence emissions differed according to shade, with the lightest shades giving the strongest emissions [41]. According to the results, the fluorescence values emitted with thicker veneers showed a decrease with respect to 0.5 mm thickness significant for G-aenial A2, and Variolink Neutral and Warm groups. This decrease can be due to the fluorescence emitted mainly by the ceramic, since the luting material does not have constituents with the capacity to overcome the emission of fluorescence by the ceramic itself. RelyX Veneer cement, regardless of its shade, maintained the lowest fluorescence values. In the present study, the composite resin Filtek Supreme XTE was used as a substrate instead natural tooth in order to avoid biological variability [27]. It is known that fluorescence in natural teeth is a multifactorial phenomenon based on multiple organic and inorganic components, age, and biotype [19; 42]. Furthermore, fluorescence is lost after extraction unless fixation procedures are performed [42]. Thus, to replicate this optical property artificially, Filtek Supreme XTE was selected due to previous papers revealing an optimal fluorescence similar to the natural tooth [42; 43; 44]. Fluorescence makes the teeth look brighter and whiter in daylight [45]. Therefore, fluorescence appears in UV light above all, but ambient light is also relevant, because it influences the color of restorations. In our study, UV light has been used, but ambient light can also induce a certain degree of fluorescence, and that is why it has been evaluated in different studies [17; 18; 27; 32; 33; 34; 35]. The peak of fluorescence emission intensity was determined to be around 450 nm. In the visible light spectrum, this wavelength corresponds to the blue color. The blue complement given by the fluorescence present in the cements should be taken into account not only by the restorative dentist, but also by the laboratory technician. Thus, the communication between the dental office and the laboratory should also indicate the type of cement that is going to be used. Natural teeth have fluorescence intensity peaks that are located at wavelengths of 350, 360, 405, 410, and 440 nm [46], which is in agreement with the obtained results, where the materials under this study present two peaks of fluorescence emission intensity around 450 nm and at 542 nm, and according to previous reports [22; 47; 48; 49; 50]. This indicates that the materials used in the present study may contain similar elements with fluorescence emission capacity, but in different percentages. Limitations of the present study include the use of only one ceramic and specific brands of resin cement, so the behavior may vary with other materials. Further studies are recommended to assess the change of the emission of fluorescence with other ceramics and resin cements.