Effects of resin composite composition and irradiation distance on the performance of curing lights

This study determined the effect of using five resin composites and two irradiation distances to test the performance of dental curing lights. Three types of curing lights with similar spectral distributions, but each delivering a different power density, were used for irradiation times ranging from 3 to 60s. Power densities were measured at 2 and 9mm from the tip of the light guide. Five composites 1.6mm thick and of the same shade were irradiated at 2 and 9mm from the light guide with energy densities of 1.2-38.0J/cm². The Knoop hardness at the top and bottom of the composite specimens was measured 15min after irradiation and again after immersion in water at 37°C for 24h. There was a linear relationship between the hardness and the logarithm of the energy density received by the composite (r²>0.81). The analysis of variance showed that the composite, the side tested, the distance from the light guide, and the curing light/irradiation time combination all had a significant effect on the hardness (p<0.01). Plots of the hardness at the bottom 15min after irradiation by each light were generated for all the composites. These plots illustrated that the effects of the different curing light/irradiation time combinations on hardness were not the same for each composite. The effects of each curing light/time combination on hardness were also different at 2 and 9mm from the light guide. In conclusion, when comparing the effects of different light sources on resin polymerization, several different composites should be irradiated at clinically relevant distances from the light guide. Using high-powered curing lights for 3 or 5s did not deliver sufficient energy to cure the 1.6-mm thick specimens of composites used in this study.