## 1 Introduction  
Resin-based composites are widely used as materials for direct restoration of teeth [1, 2]. Their advantages, however, such as easy handling and excellent aesthetics, are accompanied by significant disadvantages, namely setting shrinkage and release of potentially toxic and allergenic monomers. For many years, research has been conducted to analyse the stability of the polymer in difficult oral conditions [3, 4, 5]. Researchers are increasingly paying attention to the fact that the physicochemical properties of composite materials are determined not only by the composition of the material itself but also by the handling and setting techniques [6, 7]. These materials are highly sensitive to the restoration technique and potential cascade of errors in the clinical procedure, which ultimately leads to a filling of poor quality and an increased risk of pulp injury [8, 9, 10]. The final quality and durability of a composite filling are determined by many factors, such as the selection of the composite material for a given clinical situation, proper cavity preparation, adhesion procedure, polymerisation, and polishing [11, 12, 13]. Proper polymerisation is of critical importance. The efficiency of photopolymerisation significantly impacts a high degree of conversion and, consequently, determines the strength and longevity of the restoration [6, 14]. Studies show that the conversion rate of composite used for direct fillings is 60-75%, which leads to chemically and mechanically unstable filling [15, 16]. This may result in secondary caries, release of monomers and degradation products, pulp injury, and may ultimately lead to tooth destruction [11, 17, 18]. One of the best methods to measure the degree of composite conversion is Raman microscopy analysis [19, 20]. Since the advent of composite materials, many lamps have been used, such as quartz tungsten halogen (QTH), xenon plasma (PAC), and argon lasers. Still, in the last few years, the most popular have become light-emitting diode (LED) lights [14, 21, 22]. They are efficient, durable, and do not emit excessive heat [23]. An interesting approach to improving the quality of in vivo polymerisation seems to be the introduction of various exposure modes, such as soft-start activation mode and pulse mode. Soft start mode involves a gradual increase in radiation irradiance over time. According to the literature, this makes it possible to reduce shrinkage stresses (compared to the classic mode). This limits the formation of a marginal gap and affects the tightness and strength of the filling. Many studies have shown it is the best mode for polymerising the composite [24, 25]. The pulse mode involves a cyclical increase and decrease in radiation intensity over time. Research shows that using this mode leads to a much smaller increase in temperature in the tooth chamber, regardless of the thickness of the dentine [26]. For this reason, this mode is recommended for deep cavities [27]. A crucial measurable feature of the curing lamp is the irradiance value (measured in mW/cm\({}^{2}\)). It determines the amount of light reaching the surface of a composite filling [28]. Irradiance value depends on many factors, e.g., distance from the composite surface [29, 30], the degree of wear, or battery level [31, 32, 33]. The operator's position and hand preference (right or left) are also significant, especially in the case of posterior teeth [34]. The risk of uncontrolled movement of the light-curing unit (LCU) also increases in the case of exposure with the so-called free hand. Sometimes the dentist is not able to properly support the hand with the lamp, and the LCU changes position, usually moving away from the cavity. Numerous studies prove that the amount of light reaching the composite surface during exposure in vivo is insufficient for proper polymerisation [35, 36]. Extending the polymerisation time and increasing the lamp power do not yield the expected results.