## 1 Introduction  
Composite resins, when cured by light-curing units (LCUs), exhibit a degree of conversion ranging from 45% to 75%, depending on various factors related to the resin composition and light-curing parameters [1]. These factors include irradiance energy, wavelength range, resin shade, opacity, filler size, and loading [2]. Insufficient polymerization (IP) can adversely affect the mechanical properties of composite resins and result in the leaching of unpolymerized monomers, which may pose biocompatibility concerns [3]. The degree of polymerization (DP) of composite resins, as well as mechanical properties like microhardness, is diminished as a function of depth because a lower amount of light is reaching deeper in its mass due to scattering, absorption, and light reflection [4]. Thus, the uniform polymerization of the material throughout its depth is not achieved because of radiation, as it passes through the mass of the composite resin (CR) and undergoes scattering, reflection, and absorption, resulting in a loss of power. This leads to non-uniformpolymerization across the entire volume of the composite resin (i.e., the restoration does not exhibit consistent physical, mechanical, and chemical properties throughout its volume) [5]. Moreover, the further the composite resin (CR) layer is from the light source, the greater the energy loss becomes. This issue is particularly significant in bulk-fill composites, as they are applied in a single layer rather than by using the layering technique (successive thin layers) [6]. Consequently, the depth of cure (DoC) gains even greater clinical importance in these cases [7]. DoC refers to the minimum acceptable level of polymerization required to adequately cure the material. Also, DoC can be assessed using direct methods such as FTIR and Raman spectroscopy [1] or indirect methods such as the scrape-back length technique [2] and microhardness testing [3,4]. A bottom-to-top microhardness ratio of 80% is commonly used as a threshold to indicate adequate DoC for composite resins. While this ratio is somewhat arbitrary, it is based on the study by [PERSON] et al. [4] and has gained widespread acceptance in the field [5; 6; 7; 8; 9]. In regular CR, this threshold is typically achieved with increments up to 2 mm. However, in bulk-fill composites, which are cured in larger increments of up to 4-5 mm, achieving an adequate depth of cure becomes even more critical [10,11]. Historically, camphorquinone (CQ) and a tertiary amine have been the primary photointiator systems used in light-cured composite resins, with CQ exhibiting an absorption peak around 460 nm in the blue light spectrum [12]. However, CQ's yellow color limits its use in highly aesthetic applications, prompting the introduction of alternative photointiators such as phenylpropanidone (PPD) and Lucirin-TPO (TPO), which absorb light in the violet or ultraviolet B (UVB) range [13]. While halogens and plasma are LCUs that emit a broad spectrum of light between 375 nm and 510 nm and can effectively cure resins containing these photoinitiators, they have drawbacks such as decreased irradiance over time, heat generation, and short bulb lifespan [14,15]. To address these issues, light-emitting diode (LED) LCUs were developed, offering narrow-spectrum emission in the blue region (450-470 nm) with high radiant power, effectively polymerizing resins containing CQ as the sole photoinitiator [16]. However, the efficacy of single-peak LED LCUs is limited for resins with additional photoinitiators like PPD, TPO, and Ivocerin, as they require broader spectrum light for activation. Unfortunately, manufacturers often provide limited information on the photoinitiators used in their composite formulations, complicating the choice of optimal curing light [17]. The advent of polywave LED LCUs aimed to overcome these limitations by emitting both blue light and violet/UVB light to activate multiple photoinitiators. However, studies have shown that polywave LEDs may not consistently outperform monowave LEDs [12; 13; 14].