and Aim: Excessive heat produced while curing of light-activated dental restorations may cause irreversible damage to dental pulp. The Aims of this study wereto measure the temperature rise(TR) induced by two different light curing units and to evaluate the relathionship between the measured TR in dentin discs of different thicknesses and different distances of light guide tips from dentin disc. Materials and Methods: In this experimental study, Quartz-Tungsten-Halogen (QTH) (400) and

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... ting Diode (LED) systems (1500) were used as the curing units. Dentin discs of 0.5, 1.0 and 1.5 mm thicknesses were prepared. Distances of 1,2 and 3 mm from the tip of the light guide and dentin discs were set. Temperatures were recorded using a digital laser thermometer. Data were statistically analyzed using two-way ANOVA, Duncan and T-test. Results: Results showed that both dentin thickness and distance between tip of curing units and dentin disks had significant effects on temperature rise in both curing units (p<0.001). Conclusion: LED produced the highest and the QTH the lowest TR for all tested conditions. Reduced thermal insults were related to increased dentin thickness, increased distance between the light guide tip and dentin, and decreased energy emitted from the light-curing unit.