The setting reaction of contemporary resin composites is activated by visible light produced from curing units. A wide variety of curing units is available on the dental market, among which the light emission diodes (LED)-based curing units representing state of the art of dental photocuring are recommended for use with modern resin composites. The LED curing units differ by emission spectra, which is the foundation for their general division into narrow-spectrum (monowave) and wide-spectrum (polywave). The curing units from monowave group produce blue light which is efficient for light-curing of composites containing camphorquinone. Most of the resin composites on the dental market nowadays pertain to this group. On the other hand, polywave curing units emit in both violet and blue wavelength range, thus being beneficial for curing of resin composites with alternative photoinitiators. The absorption spectrum of these alternative photoinitiators is typically shifted towards lower wavelengths, rendering blue light produced by monowave units less effective for their activation. According to clinical handling and application, resin composite materials can be divided into conventional and bulk-fill. While conventional reins composites require an application and separate curing of layers which should not exceed 2 mm in thickness, bulk-fill resin composites enable making the whole restoration in a single layer which can reach 4-5 mm in thickness. Most of the bulk-fill resin composites contain classical photoinitiator system based on camphorquinone and tertiary amine, whereas some materials employ additional violet light-absorbing photoinitiators. The aim of this study was to assess the effect of light curing using monowave and polywave curing units on the following properties of conventional and bulk-fill resin composite materials: light transmission, microhardness, and temperature rise during polymerization. Materials and methods Three conventional resin composites (Tetric EvoCeram, Gradia Direct Posterior, and Grandio) and three bulk-fill resin composites (Tetric EvoCeram Bulk Fill, X-tra Fil, and Filtek Bulk Fill) were investigated. Discoid resin composite specimens (diameter = 6 mm, height = 2 and 4 mm) were light cured using a monowave curing unit (Bluephase Style M8) or a polywave curing unit (Bluephase Style). Light transmission through resin composite layers of 2 and 4 mm thickness was measured during light curing in real-time using a charge-coupled device visible light spectrometer. Light transmission was calculated as a fraction of incident curing light intensity which was transmitted through resin composite specimens. Real-time light transmission curves were used to describe polymerization kinetics and to determine the time of polymerization completeness. The temperature rise during light curing was measured in real-time using a T-type thermocouple at the bottom of resin composite layers of 2 and 4 mm in thickness. The specimens for microhardness measurements were stored for 24 hours at 37 °C prior to testing, in order to complete the post-cure polymerization. Microhardness was assessed using a Vickers test at specimen surface and at the bottom of 2 and 4 mm thick resin composite specimens. On each specimen, three indentations were performed using a load of 100 g and dwell time of 15 seconds. The ratio of microhardness at a certain depth and surface microhardness was used as an indicator of polymerization effectiveness. Statistical analysis was performed using a three-way analysis of variance (ANOVA), mixed model ANOVA, and Pearson correlation analysis. Partial eta-squared statistics were used to determine the practical significance of each independent factor from three-way ANOVA. Tukey post-hoc adjustment was used for multiple comparisons. The level of significance for all statistical analyses was set at 0.05. Results The greatest effect on light transmission was exerted by layer thickness and resin composite type. Significantly lower light transmission values were measured for conventional resin composites (0.7 ‒ 16.7%) compared to the group of bulk-fill resin composites (2.9 ‒ 27.0%). The influence of curing unit type was statistically significant but with lower effect size compared to layer thickness and material type. An increase in light transmission of various extents was observed during polymerization in all resin composite materials. The curves obtained by plotting the light transmission as a function of time were used to describe polymerization kinetics and to develop a method for assessing the time of polymerization completeness. For clinically relevant layer thickness (2 mm for conventional and 4 mm for bulk-fill resin composites), the time of polymerization completeness ranged between 15.3 and 42.1 seconds. The strongest effect on the temperature increase during light curing was exerted by layer thickness, which was followed by the type and intensity of the curing light. Curing with a monowave unit resulted in a temperature rise of 4.4 ‒ 7.0 °C, whereas specimens cured using polywave unit showed a temperature rise of 5.5 ‒ 9.2 °C. Microhardness values were the most strongly influenced by material type, followed by layer thickness and curing unit type. Statistically significant differences in microhardness values among layer thicknesses of 0, 2, and 4 mm were more frequently identified in conventional resin composites, while bulk-fill resin composites maintained more than 91% of their surface microhardness at 4 mm thick layers. Conclusions Light transmittance, microhardness, and temperature rise during polymerization were most strongly affected by layer thickness and differences in material composition. Although the effect of curing light type on the evaluated properties was formally identified by statistical analysis, the effect size was small compared to other factors, leading to the conclusion that there was no clinically relevant benefit in using polywave over monowave light for the curing of conventional and bulk-fill resin composites evaluated in this study.