## 4 Discussion
### Differences in Light-Curing Units (LCUs) and Depths of Cure (DoCs)  
[PERSON] et al. [60] and [PERSON] et al. [61; 62] reported that polywave LED LCUs often exhibit non-uniform irradiance profiles at their tips, resulting in inconsistent polymerization depending on the photoinitiator and the orientation of the LCU. In contrast, monowave LED, halogen, and plasma arc LCUs tend to provide more uniform light emissions. The Valo polywave LED LCU, when emitting light of 20 J/cm\({}^{2}\) in total, was calculated to have a radiant emittance transmitted of 15.5 (+/\(-\)0.4) J/cm\({}^{2}\) over 420-495 nm and of 4.5 (+/\(-\)0.2) J/cm\({}^{2}\) over 380-420 nm [43; 44]. Flowable bulk-fill composites, such as SDR, Filtek Bulk Fill, Xtra-Base, and Venus Bulk Fill, are expected to exhibit a higher depth of cure than high-viscosity composites, such as SonicFill and Tetric EvoCeram Bulk Fill. In the present study, the high-viscosity SonicFill and Tetric EvoCeram Bulk Fill did not present 80% of the top hardness at the 4 mm depth when irradiated with the Valo and QTH LCU, respectively. This could be because the QTH LCU emitted light with total energy of 12 J/cm\({}^{2}\), which is much less than the 20 J/cm\({}^{2}\) that the rest of the LCUs emitted. Nevertheless, when SonicFill was irradiated with the Valo LCU, it received 20 J/cm\({}^{2}\) of energy, but only the blue spectra could polymerize it, and thus just 15.5 J/cm\({}^{2}\) of the energy absorbed was able to be useful for polymerization. Tetric EvoCeram Bulk Fill, containing CQ, TPO, and Ivocerin as photoinitiators, did not fulfill the 80% microhardness criterion at the 4 mm depth when irradiated by the polywave LED LCUs, but did when irradiated by QTH or monowave LED LCUs. When the QTH or monowave LED LCU was used for Tetric EvoCeram Bulk Fill, the 80% criterion was fulfilled even for the 5 mm depth. These findings could be attributed to the fact that the violet spectra of light emitted from the polywave LED LCUs were not able to penetrate to the 4 mm depth, but only up to 2.5 mm depth [41; 63; 64; 65; 66], especially in viscous composites, despite its high light translucency. Nevertheless, QTH LCUs emit light in a broad spectrum, and may adequately cure bulk-fill composites with additional photoinitiators. On the contrary, blue light, emitted by QTH and monowave LED LCUs, was able to penetrate up to the depth of 4 mm or deeper, and was able to adequately cure the Tetric EvoCeram Bulk Fill. The QTH used (Eilpar Trilight) delivers a small amount of energy, about 10%, in the 350-420 nm (violet) range, while the Bluephase G2 and Valo deliver 14-24% and 17-19% in this region, respectively [41; 63; 66]. Therefore, even the QTH LCUs can emit light in the violet spectrum, and this could be the explanation for the adequate polymerization of Tetric Evo Ceram Bulk Fill with the QTH LCU. No advantages of polywave LCUs over monowave ones were reported in other studies either [41; 60; 61; 62; 63; 64; 65; 66; 67; 68]. Monowave LED LCUs like Demi Ultra outperformed polywave LCUs in curing composites such as Tetric EvoCeram Bulk Fill, achieving 80% of the VHK microhardness ratio at greater depths. Flowable composites like SDR and Filek Bulk Fill showed a better curing depth compared to high-viscosity materials like SonicFill and Tetric EvoCeram Bulk Fill, which struggled to reach 80% hardness at 4 mm with Valo and QTH LCUs due to limited blue light penetration. Mixed results have been reported for monowave versus polywave LCUs. [PERSON] et al. [33] found no significant difference in the degree of conversion (DC) between monowave and polywave LCUs, but others noted that irradiation time and intensity are critical. [PERSON] et al. [10] found no improvement with quad-spectrum LCUs, while [PERSON] et al. [52] reported microhardness drops with high-intensity LCUs. Longer irradiation times led to better curing depths [64], while high irradiance with short exposure proved inadequate [50; 69; 70; 71; 72].