rEsuLts Analysis of the data revealed significant (P<.05) differences among the samples cured by the light sources in terms of Vickers hardness, depth of cure, and thermal changes during irradiation. Hardness The hardness values of the 0 mm, 1 mm, and 2 mm levels all indicated significant differences be- tween the samples cured by the TPB light unit and those cured by the EMS light unit (regardless of the 4 s or 6 s exposure time). Note that hardness values of the 3 mm and 4 mm levels were mea- surable only in those samples cured by the TPB light unit. Indeed, the maximum Vickers hardness values were produced by the TPB light unit, which gave 81.25, 73.29, 65.94, 55.83, and 24.53 for the 0 mm to 4 mm levels, respectively. The minimum hardness values were produced by the EMS light unit with a 4 s exposure time, which gave 73.27, 61.51, and 31.59 for the 0 mm to 2 mm levels, re- spectively. Between the samples irradiated by the EMS light unit for 4 s and 6 s, there was no signifi- cant difference in the hardness value at the 0 mm or 1 mm level, but there was at the 2 mm level. In samples cured with the TPB light unit, the 1 mm and 2 mm levels attained hardness values greater than or equal to 80% of the corresponding surface hardness values. In samples cured with the EMS light unit for 4 s, only the 1 mm level attained 80% of the corresponding surface hardness values. In samples cured with the EMS unit for 6 s, the 2 mm level attained hardness values that were closer to 80% of the corresponding surface hardness val- ues than in samples cured for only 4 s (Table 1, Figure 1). Mousavinasab, Meyers Depth of cure The depth of cure (DOC) reached its maximum of 2.64 mm in samples cured by the LED light unit and its minimum of 2.00 mm in those cured by the QTH light with a 4 s exposure time. There was thus a significant (P<.05) difference in DOC between samples cured with the LED light unit and those cured with the QTH light unit. However, this signif- icant difference was also found between samples cured with the EMS light unit for 4 s or 6 s of ex- posure time, since the DOC was 2.00 mm or 2.15 mm, respectively. Thermal changes There was only an insignificant (P>.05) differ- ence between the mean temperature rise gener- ated by a 40 s irradiation with LED light (1.87 °C, SD=0.34) and that generated by a 4 s irradiation with QTH light (1.88°C, SD=0.19). However, for a 6 s irradiation with QTH light the analogous differ- ence (3°C, SD=0.27) was considerable. Note that the difference in temperature rise between 4 s and 6 s of irradiation with the QTH light unit was also significant. dIscuIon In this study, the curing performance of a high power halogen light unit (EMS) was matched against that of a second-generation LED light unit (TPB) with the aim of determining whether the halogen light unit with its short recommended exposure time is capable of curing composites as completely as the LED light unit with its 40 s ir- radiation time. In general, larger hardness values are indi- cators of more extensive polymerization.22 The depth of cure for light activated dental resin com- posites has thus often been evaluated indirectly by measuring the hardness of the material at specific depths.23,24 It has been suggested that the depth of cure be defined as the level above which the hardness value of the cured resin composite is greater than or equal to 90% (or recently 80%) of the surface hardness value.25,26 The resin composite used in this study contains camphorquinone as the photoinitiator, and gener- ally such resin composites can be more efficiently Figure 1. Vickers hardness values of resin composite at different intervals cured by different light sources and exposure time. Table 1. Vickers hardness values in different intervals and DOC of resin composite cured by light sources. SD is given in parenthesis. Intervals TPB EMS (4s) EMS (6s) July 2011 - Vol.5 0 mm 81.25 (1.61) 73.27 (1.17) 73.74 (0.78) 1 mm 73.29 (0.88) 61.51 (0.84) 62.16 (1.02) 2 mm 65.49 (1.00) 31.59 (0.39) 53.52 (1.01) 3 mm 55.83 (1.40) - - 4 mm 24.53 (6.05) - - DOC (mm) 2.64 2 2.15 301 European Journal of Dentistry