Surface microhardness of a resin composite: a comparison of a tungsten halogen and a LED light curing unit, in vitro

A comparison has been made between published surface microhardness numbers (VHN) of a commercial resin composite for different exposure times to a quartz tungsten halogen (QTH) and light-emitting diode (LED) light-curing unit (LCU). Both LCUs produced comparable hardness at both top and bottom surfaces, respectively, and similar bottom/top hardness ratios, for a specimen thickness of 1.5 mm, given sufficient exposure time (40 s) and an elapsed time of 24 h before measurement (for hardness numbers). However, some data are significantly different. There is no advantage in either LCU regarding optimal hardness and hardness ratios given an appropriate protocol. Immediate finishing (1 h) was more appropriate to the use of the LED LCU (with adequate exposure time). The effect of elapsed time after exposure on microhardness was more pronounced with the QTH LCU.