## 1 Introduction  
As a result of technological innovations in dental composite materials and light-curing units (LCUs), advances in filling placement techniques, and increased aesthetics for patients, light-cured resin-based composites (RBCs) have become the most widely used restorative materials today [1; 2; 3; 4; 5]. Many RBCs only have camphorquinone (CQ) as a photoinitiator due to its broad absorbance range of 370-500 nm and peak absorbance at 468 nm in the visible light spectrum [6]. Any LCU with reasonable irradiance in this region can be used for curing RBCs [7]. Although rapid curing may be attractive to the practitioner, especially in deep cavities and large restorations, the radiation heating produced by high light intensity light-curing units (LCUs) such as argon-ion lasers and plasma-arc curing (PAC) lamps can cause unnecessary and unacceptable temperature increases in the tooth, increasing the risk of pulp damage [6; 8; 9; 10]. Despite their popularity, solid-state light-emitting diode (LED) LCUs have been produced to overcome some of the limitations of quartz-tungsten-halogen (QTH) LCUs [11; 12; 13; 14; 15]. Instead of the hot filaments used in halogen bulbs, LEDs use junctions of doped semiconductors (p-n junctions) to produce blue light [16; 17; 18; 19]. Since LEDs have no emission in the infrared range, LEDs are expected to produce less irradiation-induced heat during the photopolymerization of RBCs and reduce the polymerization stress levels that develop at the composite-tooth interface [15; 20; 21]. But, because the relatively low power irradiance of first-generation LEDs results in the insufficient polymerization of RBCs and poor mechanical properties of restorations, \"second-generation\" LEDs have been produced [17; 18]. New LEDs use a single LED with a larger semiconductor crystal, producing much higher light intensity (above 500 mW/cm\({}^{2}\)) [18]. However, first- and second-generation LED lights use only one type of LED (monowave, or single-peak technology) and their peak emission coincides with the maximum absorption of CQ. Furthermore, their narrow bandwidth of emitted radiation is unable to cure lighter RBCs containing certain co-initiators (PPD, BAPO, TAPO) that absorb light energy in shorter wavelength regions (UV-A) of the visible light spectrum (410 nm) in addition to CQ [16; 18; 22]. Therefore, third-generation LED units based on polywave (dual/multipeak) technology with high-power LEDs (providing light intensities of up to 2000 mW/cm\({}^{2}\)) have been developed [2; 8]. Since these LED units use a combination of blue and ultraviolet LEDs to produce a wide spectral output, they may be able to polymerize RBCs with a wider range of photoinitiators (e.g., PPD, Lucirin TPO) [19; 22]. However, there are very few RBCs that use these photoinitiators, and severe heat generation becomes a clinical concern for gingival and pulpal tissues when using power LEDs [8]. Therefore, low-intensity second-generation LED LCUs are generally considered adequate to polymerize RBCs, unless specifically required [2; 8]. Studies have shown that the most important clinical problem with RBCs is limited polymerization, which plays an important role in the final success of the restoration as it is closely related to the material's physical and mechanical properties such as surface hardness, wear resistance, water absorption, and color change, as well as its biocompatibility due to the residual monomer content [23]. Unreacted components might leak from the restoration, possibly resulting in recurrent caries, pulpal irritation, and local tissue reaction [12; 24; 25; 26; 27]. Residual monomers in crosslinked polymeric matrix systems can be detected by the degree of polymerization (monomer conversion) [24; 28]. The degree of conversion (DC) is defined as the percentage of reacted C=C bonds (aliphatic) to the unreacted C-C bonds (aromatic) [24; 28].