## 1 Introduction
The procedural characteristics mentioned can affect the aesthetics of an aligner in a positive or negative way. The aligners currently on the market are distinguished according to the thermoforming materials, some of those used are polypropylene (PP), polycarbonate (PC), thermoplastic polyurethanes, and copolyester, the most used being polyethylene terephthalate glycol (PETG), and many others. Aligner thickness tends to vary between 0.50 to 1.50 mm. Like with their manufacturing material, this can impact their chemical properties, and, therefore, their optical characteristics, already affected by masticatory stress, salivary enzymes, and food coloring during their 14 consecutive days of wear [13; 14; 15]. From an aesthetic point of view, since each template should last two weeks, the transparency of the aligner should be maintained during this period to ensure that the patient has a palatable option [16; 17]. The optical properties of the aligner are determined by spectrophotometric transmittance. It is usually expressed as percentage transmittance (%T) and is defined as the percentage of light that can be transmitted from the other side of the surface, so the greater the transmittance the more transparent the material [11]. The first study conducted in 2009 with the aim of investigating aligner aesthetics at this term is limited to only taking the absorbance values of a single brand into consideration [15]. In 2015 [PERSON] et al. analyzed for the first time the different levels of transmittance and absorbance of various models of clear aligners, selected from three different manufacturers: All in (Micerium, Avegno, GE, Italy), Invaslign (Align Technology, Santa Clara, CA, USA), and F22 (Sweden and Martina, Due Carrare; PD, Italy). Of those analyzed, F22 was significantly more transparent, before and after aging. It emerged that around 80% of light goes through the F22 aligner, compared to 70% for Invisalign aligners and 55% for All in [18]. In 2022 [PERSON] et al. examined the transmittance and absorbance of 21 clear aligners from six different manufacturers: All in (Micerium, Avegni, GE, Italy), Air Nivol (NIVOL SRL, Navacchio Cascina; PI, Italy), Arc Angel (Deckra Group Srl, Modena, MO, Italy), Invisalign (Align Technology, Santa Clara, Ca, USA), F22, F22 EvoFlex (Sweden & Martina, Due Carrare, PD, Italy), and Nuvola (G.E.O. UK, European Orthodontic Ltd.). F22 and F22 EVoFlex resulted significantly in better absorbance and transmittance than the others [19]. Electromagnetic radiation in the range of visible light has wavelengths from 400 to 700 nm, but the human eye evaluates the intensity corresponding to the various wavelength to different extents. In fact, the higher wavelengths (near 700 nm) are poorly absorbed by the visual pigments; the lower wavelengths (near 400 nm) can be absorbed by the visual pigments of the eye, but do not reach the retina because they are absorbed by the cornea and lens. The peak of the highest human spectral sensitivity measured is at about 560 nm, the yellow-green color region. Currently, literature studies correlating how the type of 3D printer and different thermodroming disks could influence aligner aesthetics are insufficient. The aim of this study is to appraise how transparency is affected by printers and thermoforming disks. Hence, we set out to compare the optical properties of 18 aligners, produced with three3 different printers while using two different materials.