## 4 Discussion  
According to the results of the present study, the research hypothesis was rejected because the emission intensity of fluorescence of feldspathic ceramic restorations is influenced by the shade of resin-based material, as well as by the thickness of feldspathic veneer. In order to achieve a natural looking restoration, the restorative materials should imitate the optical properties of the natural tooth, including fluorescence [15]. The wavelength of the excitation beam was 360 nm, which is the wavelength that causes a peak emission fluorescence intensity of the tooth. Ceramic veneers are usually fabricated as thin layers; therefore, the type and color of the cementing agent could influence the fluorescence of the restoration, as well as the final result [27]. Different levels of fluorescence were detected among groups tested. It is evident that the high translucency of the feldspathic ceramic allows the fluorescence of the luting agent to influence the final fluorescence of the restoration. Hence, the fluorescence values of restorations are the result of combination of the fluorescence of feldspathic ceramic and the underlying cement filtered by the ceramic [32]. In the present study, only one type of ceramic has been evaluated. Although fluorescence of dental ceramics have been previously reported [18; 21; 27; 32; 33; 34; 35], literature about this optical property in current ceramics is scarce [21; 27] and, as far as we know, there are no data about the ceramic (CEREC Blocs) used in this study. Feldspathic ceramic is characterized as an extremely aesthetic material, indicated to mimic the dental structure [36]. The ceramic used in this study was a CAD-CAM feldspathic ceramic, available for digital manufacturing, and sintered by optimized industrial procedures, which results in blocks with fewer flaws and pores, and better mechanical properties than the traditional or hand-built ceramic [37; 38]. Basic components of ceramics and dental resins are not able to produce fluorescence, so this can be achieved by incorporating rare-earth oxides--such as europium, terbium, and cerium--which have a strong fluorescence when exposed to UV light [17; 19; 33; 39]. Other procedures, such as the application of an external fluorescent glaze layer on a pressed lithium disilicate ceramic, have been recently reported [21]. Regarding composite resin, manufacturers do not disclose the exact composition of these materials, although it is known that some luminescent species--such as rare-earth oxides, terbium coordination polymers of PEMA, or aromatic complex--have frequently been used [19; 22; 24]. Figure 3: Mean fluorescence values (millions of arbitrary unit, a.u.) and standard deviation bars obtained for bonded ceramic samples according to the resin-based luting agent and the feldspathic ceramic thickness (n = 10). Despite the fact that fluorescence is one of the optical properties of natural teeth that has attracted the attention of dental professionals in recent years [19], papers on this subject are limited. [PERSON] et al. [28] also quantified the fluorescence of two different ceramic veneers (lithium disilicate glass-ceramic and fluorapatite glass-ceramic) and two types of resin cements (light-curing versus self-adhesive resin cements). However, the difference in materials and methodology does not allow a direct comparison of the results. With ceramic veneers of 0.5 mm thickness, the groups in which Variolink Veneer and the flowable composite resin G-aenial were used, exhibited similar fluorescence values to the group used as reference (preheated Filtek Supreme XTE). In contrast, the fluorescence intensities were the lowest when RelyX Veener resin cements were used. These results might be explained by the luminophore content of the cements evaluated. In the case of Variolink Veneer, it contains ytterbium trifluoride, a compound that provides fluorescence [40].