## 1 Introduction  
At first, the real discriminant in the choice of composite resin for orthodontic bonding was the mechanical resistance to stress induced by orthodontic forces, especially in critical clinical cases [1; 2; 3; 4]. An ideal orthodontic adhesive system, in fact, is one that is able both to resist the orthodontic-chewing forces during all phases of treatment and to preserve the integrity of the dental enamel following the removal of the bracket [5]. The bond strength of the bracket to the enamel is strictly influenced by each step of the application process, such as acid etching and drying time, the method of application of the adhesive and the curing time [6]. In recent years, optical properties (color, translucency, opalescence and fluorescence) and translucency in particular, have become fundamental properties to be evaluated for choosing the right composite resin, not only for bracket bonding procedures [7]. More recently, treatments with transparent aligners must involve additional auxiliaries, such as attachments in composite materials, which, acting as force transducers, make the orthodontic movement more effective, thus improving the biomechanics while respecting aesthetics [8]. Translucency, together to color, is the most easily observed optical property of materials that describes the partial passage of light through a certain structure [7]. The main factor that determines the optical properties of a material is its chemical composition: it is for this reason that the refractive index is different between dental biomaterials such as ceramics, glass ionomer cements and composite resins [9; 10; 11; 12; 13; 14; 15; 16; 17]. For composite resins, some of the factors responsible for the optical dispersion of light throughout the material are the overall composition and the filler content, both in terms of shape and size [18; 19]. Most orthodontic adhesives consist of composite resin. Though, the orthodontic adhesive systems represent a variant, in terms of formulation, to the adhesives and composites used in conservative dentistry, differing from these mainly in the increased concentration of co-monomers found in their basic formulation [6]. The orthodontic composite resin is a complex material consisting of an organic matrix, (Bis-GMA, UDMA, TEGDMA), synthetic polymers (ceramic reinforcing fillers and silane coupling agents) and molecules that promote or modify the polymerization reaction. Resin adhesives consist of methylacrylates (e.g., hydroxyethyl methacrylate or HEMA, and 4 methacryl-oxyethyl-trimellitic-anhydride, or 4-META), dimethylacrylates (e.g., triethylen-glycol dimethacrylate, or TEGDMA, and Bis-GMA, consisting of a molecule of Bisphenol A and two molecules of glycidyl methacrylate), phosphonated pentacrylic esters, acrylic amides, aldehydes (for example glutaraldehyde) and organic acids [20]. Bis-GMA, together with UDMA (urethane dimethacrylate), is the main constituent of the resin matrix of composites; one of the most important characteristics of this dimethacrylate is its high viscosity, which makes it necessary to use a more fluid resin to dilute it. TEGDMA and HEMA have lower viscosity and can be considered diluting monomers. Resin adhesives also contain solvents, such as ethanol, acetone or water, which act as a vehicle for water between the collagen fibers affected by the adhesive bond, preventing their collapse. If not evaporated properly, however, these substances can negatively affect the quality of the adhesion [20]. In orthodontic resins, the inorganic fillers added to the organic matrix act by modifying the mechanical behaviour of these adhesive materials, conferring reinforcement and stiffness, decreasing dimensional variations and improving their versatility during clinical use [21]. In recent decades, resin composites have generally been classified on the basis of the size of the particles making up the filler, distinguishing themselves into: hybrids (8-30 \(\upmu\)m), microhybrids (0.6-3.6 \(\upmu\)m) and microfill (approximately 0.04 \(\upmu\)m) [22; 23].