###### Abstract  
(1) Background: The shade of resin-based materials and ceramic thickness influence the optical color of laminate restorations. The purpose of this study is to evaluate--in vitro--the effect of resin-based cement shade and ceramic thickness on fluorescence of feldspathic laminate venereis; (2) Methods: 180 samples of feldspathic ceramic A2 shade with two different thicknesses (0.5 and 0.8 mm) were obtained. The samples were cemented to composite resin substrates with one of the following materials in different shades (n = 10): resin cement (Variolink Esthetic in Light, Neutral and Warm shades; or RelyX Veneer in B0.5 /white, Translucent and A3 Opaque/yellow opaque shades); flowable composite resin (G-aenial Flo in A2 and A3 shades) or a pre-heated composite resin (Filtek Supreme XTE, A3 body shade). The fluorescence spectra were obtained by means of a spectrofluorometer. Two-way ANOVA, Tukey, and Student's \(t\)-tests were performed (\(\alpha\) = 0.05); (3) Results: Fluorescence values were significantly influenced by the resin-based agent tested (\(p\) < 0.001), the thickness of ceramic (\(p\) < 0.001), and their interaction (\(p\) < 0.001). The lowest fluorescence values were achieved by RelyX Veneer resin cement regardless its shade and the ceramic thickness; (4) Conclusions: both the shade of resin-based agent and the feldspathic ceramic thickness influenced the fluorescence of laminate restorations. Keywords: feldspathic ceramic; resin cement; flowable resin; fluorescence; thickness +
Footnote †: journal: Applied  
providing three times more fluorescence than enamel [11; 20; 22; 23; 24]. Fluorescence can be classified as distinctive clinical optical property that not only makes teeth appear whiter but also brighter by emitting more blue radiation due to fluorescence, which converts UV radiation (invisible to the eye) to blue radiation (visible to the eye). Thus, to provide better integration, it is mandatory that teeth restored with ceramic veneers have a fluorescence emission similar to that of the natural teeth [19]. The type and composition of the ceramic will influence the intensity of fluorescence [19]. Some luminescent additives--such as europium and other rare-earth elements that exhibit visible fluorescence--are included in composition of ceramics and resin cements to obtain fluorescence properties similar to the tooth structure [19; 25; 26; 27; 28]. To date, few studies have evaluated the fluorescence of ceramic restorations. [PERSON] et al. [27] evaluated the impact of tooth substrate shade on color differences, transmittance, and fluorescence of CAD-CAM (Computer Aided Design and Computer Aided Design Manufacturer) leucite based ceramics. The association of ceramic samples with darker substrates decreased fluorescence intensity. [PERSON] et al. [28] showed that the apparent fluorescence of laminate veneers was influenced by the combination of two different ceramic veneers and the cement (light-cured or self-adhesive dual resin cements). Other authors also showed that high-fluorescence resin-based cement may interfere with the final esthetic result of thin restorations [20]. Light-cure resin cements are indicated when luting relatively thin and translucent restorations, as it allows light irradiance to activate the photo-initiators [29]. This type of resin cement exhibits clinical advantages such as long period of working time, setting on demand, and better color stability [30]. Recently, there has been an increasing trend to use flowable composite resins as light-cure cements for adhesive luting [29] in order to benefit from their physical properties (more filler-loaded than resin cements), as well as an improved cost-benefit compared to resin cements [31]. Several devices and methods for analysis of fluorescence in aesthetic materials have been employed in previous papers. Fluorometers or spectrofluorometers are commonly used because they provide quantitative results without the limitations of photography methods [19; 27].