## 4 Discussion
However, [PERSON] et al. reported that smaller energy levels (5-15 J/cm\({}^{2}\)) were needed to produce relatively high conversion levels at the top or bottom surfaces of 2 mm thick specimens [37]. Within a narrow range, by using the same energy level (intensity \(\times\) time) or a fixed amount of LED light energy with a low-intensity curing unit, it is possible to achieve the same and optimal DC (as it is with the high power density LCU) up to a certain sample thickness (2 mm) if the curing time is enough [37]. This result is in agreement with the result of the DCs for packable Filtek P60, micro-hybrid Filtek Z250, and ormocer Admira placed with the standard technique. The optimal DC values were obtained for up to 2 mm with small total energy levels (-4.6 for Filtek P60 and Filtek Z250 and -9 J/cm\({}^{2}\) for ormocer Admira, respectively). [PERSON] et al. (2003) also stated that by increasing the RBC thickness above 2 mm but not exceeding 6 mm, energy levels exceeding about 30 J/cm\({}^{2}\) were needed to achieve acceptable levels of DC for Filtek Z250 [37]. This result partly contradicts this study, as acceptable DCs were achieved for packable Filtek P60, micro-hybrid Filtek Z250, and ormocer Admira placed with the bulk technique up to 4 mm depth, despite the fact that they were polymerized by a lower total amount of light energy from the LED LCU (-4.6 and -9 J/cm\({}^{2}\), respectively) compared to 30 J/cm\({}^{2}\). However, the same RBCs placed with the incremental technique showed lower DC values up to 4 mm compared to the same RBCs placed with the bulk technique, despite their higher energy levels (-18 and -37 J/cm\({}^{2}\), respectively). This result conflicts with [PERSON] et al. (2003) [37]. The results of this in vitro study were found to be clinically significant. The optimal DC values were obtained for packable Filtek P60, micro-hybrid Filtek Z250, and ormocer Admira placed with standard, bulk, and incremental techniques and polymerized with the second-generation LED LCU with the recommended time, up to 8 mm depth, in this study. The results indicated that the type of RBC and placement technique had a significant effect on the DC values. It may be concluded that the bulk placement technique offers a good alternative to the incremental placement technique, especially in deep restorations. This study also showed that the second-generation Hilux LED LCU did not polymerize the hybrid Tetric Ceram and flowable Tetric Flow sufficiently with the recommended times, even at 2 mm depth, and the flowable composite showed a much lower DC in comparison with the hybrid, ormocer, and packable composites. This result is not surprising for TetricFlow, considering the differences in the filler loading and filler types used because it has less filler. However, it was found surprising for hybrid Tetric Ceram. On the other hand, it is unknown whether hybrid Tetric Ceram and flowable Tetric Flow contain co-initiators or not. It is thought that the reason for the inadequate degree of polymerization by the second-generation LED LCU might be their co-initiators. Thus, if the LED LCU is used, the dental RBC should be carefully selected, since RBCs containing co-initiators need shorter wavelengths for polymerization. Otherwise, LED curing may be less effective. It has also been stated that the recommended exposure times are generally inadequate to optimize the DCs of the recommended increment of composites, but longer exposures are required [24]. The shade A\({}_{3}\) of RBCs was used for determining the DCs of RBCs to minimize the effects of colorants on light polymerization, because differences in light transmittance through different shades may affect their clinical properties in this in vitro study [47]. Several studies have suggested that darker shades should be polymerized for longer periods because more pigmented RBCs are the least curable and present a lower depth of polymerization [23; 34; 44].