## 4 Discussion
It was stated that if the curing time is sufficient enough, the optimal DC can be obtained using a lower light power density [37; 42]. Some RBCs polymerized with LED LCUs may require an additional curing time or smaller thickness increments [2]. In this study, RBCs were polymerized using a 229.153 mW/cm\({}^{2}\) light intensity LED LCU with curing times recommended by manufacturers, using the continuous standard mode. While optimal DC values were obtained for packable Filtek P60, micro-hybrid Filtek Z250, and armocer Admira, lower DC values were obtained for hybrid Tetric Ceram and flowable Tetric Flow. Longer exposure times might be required for polymerizing the RBCs tested to obtain optimal or higher DCs. It was also reported that the top surface DC of RBCs was independent of the light used. The top surface of an RBC easily polymerizes in a very short time, even using low power density from relatively long distances due to little light loss [43]. However, [PERSON] et al. (1998) stated that the bottom surface of the RBC is affected by reduced transmission of light through the relatively opaque RBC, although the top surface of the increment RBC might have sufficient physical properties [9]. The light intensity and its effectiveness are decreased, and a gradual reduction in the DC of the RBC monomer occurs as the depth increases or the distance from the tip of the light source to the RBC increases [23; 36; 38]. This finding is in agreement with this study. The DCs of RBCs in 8 mm thickness placed with bulk and incremental techniques diminished gradually from the top surface to the bottom surface. It has been stated that the effective polymerization of RBCs mainly depends on the distance between the LCU tip and the restoration surface [44]. Although it is recommended to place the light tip as close to the RBC surface as possible, clinically, this is often difficult to achieve in proximal restorations. The distance between the light tip and the bottom of the proximal cavity has been shown to be more than 8 mm [36]. Some studies have shown that when the restoration is built up with the bulk technique in a deep cavity, light passes through the bulk of a restorative material, and its intensity is greatly reduced due to light scattering by filler particles and the resin matrix. Thus, polymerization effectiveness is reduced as the depth increases [1; 24; 43]. However, [PERSON] et al. (2007) stated that when the restoration is built up incrementally in a deep cavity, the first increment, furthest away from the light source, receives a lower energy density than the top increment, closest to the light guide [35; 44]. It was demonstrated that this additional energy was not sufficient to enhance polymerization; even the bottom half of the sample may have received additional light energy, and the bottom surface of the RBC had the least polymerized structure [43]. These results are compatible with the DCs of the bottom surfaces of RBCs placed with bulk and incremental techniques in this study (8 mm depth). Studies have shown that narrow LCU tips that can be used inside the cavity can ensure that the deepest parts of the restoration receive adequate radiation [11; 45; 46]. It was also reported that LED light guides have a 60\({}^{\circ}\) bend in order to access all areas of the oral cavity. Significant losses of transmission occur with large-diameter light guides [13; 21]. The light guide tip of the fiber optic LED LCU used in this study for polymerizing RBCs was 11 mm in diameter and had a 60\({}^{\circ}\) bend. Its broader diameter compared to the 4 mm sample diameter might lead to significant transmission losses [46]. It is suggested that when using a broad-diameter light guide tip, a longer curing time could compensate for reduced illumination and might ensure adequate polymerization [45]. It was declared that light energy, which is calculated as the light intensity of the LCU (mW/cm\({}^{2}\)) \(\times\) irradiation time (in seconds), should be 21-24 J/cm\({}^{2}\) for adequate polymerization of a 2 mm thick RBC [8; 16].