<p>Heat transfer plays a critical role in medical and dental procedures, as excessive heat generation may cause irreversible damage to the dental pulp. Although many previous studies have focused on temperature rise as the primary thermal indicator, temperature alone does not adequately describe the heat transfer process or the actual thermal load delivered to biological tissues, as it represents only the final outcome of energy exchange. Therefore, parameters beyond temperature change, such as heat flux, must be considered to achieve a more accurate physical interpretation. The objective of this study was to estimate the absorbed heat flux in human dentin discs subjected to irradiation from different LED and QTH light-curing unit modes using an inverse heat conduction approach. In vitro dentin surface temperature was measured using an infrared thermometer. The inverse problem was solved using the conjugate gradient method implemented in a MATLAB code coupled with COMSOL Multiphysics via LiveLink, with COMSOL serving as the forward solver. Prior to the inverse analysis, the free convection heat transfer coefficient was identified to define appropriate thermal boundary conditions. The estimated heat fluxes were consistent with the irradiation patterns reported by the manufacturers while revealing notable differences between curing modes. The highest absorbed heat flux (289.5 mW·cm⁻²) was obtained in the SOFT LED mode, whereas the HIGH LED modes produced lower values (209 and 202.6 mW·cm⁻²). The lowest heat flux (154.6 mW·cm⁻²) was observed in the 40 s STANDARD QTH mode despite producing the greatest temperature rise. This discrepancy demonstrates that temperature change alone is not a sufficient indicator for evaluating the thermal impact of light-curing units, as similar or even higher temperatures may correspond to substantially different absorbed energy levels. Overall, these results show that using heat flux together with temperature rise helps to better understand heat transfer processes and avoids misinterpretations that may occur when temperature change is considered alone.</p>