How light irradiance and curing time affect monomer conversion in light-cured resin composites

We tested the hypothesis that the degree of conversion of a light-cured dental composite relates to the calculated (s x mW cm(-2) = mJ cm(-2)) rather than to the irradiance value (mW cm(-2)) of the light source. Two light-curable composite resins were cured with three different light irradiance values over different curing times. The specimens tested were 2, 4 or 6 mm thick, and the degree of conversion values were measured with Raman spectroscopy on the top and the bottom surfaces of the specimens. The highest conversion value of one of the materials was just below 60%, while the maximal conversion value of the other material was just below 65%. That difference in conversion values could be related to differences in monomer systems used in the two composites. By considering light energy per square centimeter (J cm(-2)) rather than light irradiance (mW cm(-2)), we found that equivalent energy values gave similar conversion values for a certain sample thickness. From these findings, we conclude that our experimental results support our hypothesis.