High intensity QTH light sources IntroducIno Light emitting diode (LED) light curing units are becoming increasingly popular in dental practice. Most of the first-generation LED light units were unable to cure composite resin in the manner of quartz-tungsten-halogen (QTH) light sources.1-5 Since the spectral output of the LEDs is concen- trated in the blue wavelength range, high power LED curing lights are capable of polymerizing some resins as well as, or better than, some QTH lights.6-8 LED curing units are characterized by a relatively narrow emission spectrum and lower heat generation than QTH curing units.6,9-14 There have been reports that exposure causes less temperature rise with a conventional curing light than with an LED curing light and that higher pulp chamber temperature changes are induced by high output curing units than by conventional curing units.15-16 Likewise, previous findings that had indicated less temperature rise with LED units than with QTH units have been debated in another study.17 The degree of conversion of composite resins is influenced by the spectral distribution and inten- sity of the curing light as well as the shade, opac- ity, and chemical composition of the resin-based composite. While both LED lights and quartz- tungsten-halogen lights are believed capable of curing resin-based composites, some differences are observed in the performance of the cured res- in. Moreover, both the composite material and its curing time have a significant association with the resulting degree of polymerization.18 It has been shown that LED light units, like conventional halogen light sources, are capable of curing the camphorquinone-based composites to an acceptable degree of polymerization19 and such resin composites show similar strain behav- ior whether an LED or halogen light curing unit is used to polymerize the resin composite.20 The ability to reduce exposure time by using high power LED or QTH lights may improve clinical time management.21 A very high power QTH light curing unit has been introduced into the market, claiming to cure resin composite with a thickness of more than 2 mm within a short exposure time. The aim of this study was to assess that high pow- er light unit in terms of the curing performance and the temperature rise during irradiation as compared with those for a second-generation LED light source. 300 European Journal of Dentistry MAtErIALs And MEtHods A hybrid composite resin with A3 shade (Filtek Z250, 3M ESPE, St Paul, MN, USA) was used in this study as the test material. The curing light sourc- es used in this study were a second-generation LED light unit with 860 mW/cm2 intensity (Trans- lux Power Blue, Heraeus Kulzer, Germany) and a very high power QTH light unit with 2890 mW/cm2 (Swiss Master Light, EMS, Switzerland). The light intensity of the units was checked using the built- in digital radiometer of an Optilux 501 light curing unit. A two-split aluminum mold with a semicircular column-shaped hole (4 mm in diameter and 8 mm deep) was used to prepare samples for measuring the depth of cure and the hardness. The mold was placed on a sheet of Mylar strip and then the resin was compressed to produce a flat surface before it was covered by a clear polyester strip (Matrix Tape Refill, 3M) and finally was photopolymerized. For the control group, a Translux Power Blue (TPB) was used in continuous light photoactivation mode for 40 s. Ten samples were prepared with the TPB light source. For the test group, a QTH light unit (EMS) was used in fast cure mode for either 4 s or 6 s. Ten samples were prepared with the EMS light source for each exposure time (4 s or 6 s). The cure depth of the resin was determined using a standard technique (ISO 4049:2000). Im- mediately after irradiation, the uncured material was scraped away with a spatula. The height of the cylinder of set resin was measured with a digital micrometer (Digital Cal, Switzerland) to a preci- sion of ±0.01 mm, and the result was divided by two. Vickers hardness values were determined at 1.0 mm intervals along the depth of the cured samples, on a flat surface that was parallel to the direction of the light source, using a universal in- denter (Leitz Wetzlar, Germany). A digital thermometer (Temp Alert, Dual Ther- mo, China) was used to measure the temperature rise during exposure for a cured disc of resin com- posite (Filtek Z250, 3M, USA) with A3 shade, 2 mm thickness, and 13 mm diameter. A circular mold was prepared from an elastomeric base mate- rial in order to support the thermocouple under the cured resin disc. For each curing light and ex- posure time, groups of ten measurements were performed at 30 min intervals with the room tem- perature controlled. The data were submitted to analysis of vari- ance (ANOVA) as well as Tukey’s and Student’s t tests at the 5% significance level.