High intensity QTH light sources cured using LED light units.27 From 78% to 95% A photoactivated resin-based composite can be of the light emitted by a blue LED unit is within fully polymerized at reduced light intensity while the wavelength range 450–500 nm, as opposed the final conversion value remains high. Curing to 56% for a conventional halogen unit.6,17 There- the composite with a high intensity light and short fore, only light within this range can best activate exposure time, such as when using the EMS unit, campherquinon.28 However, some studies have shortens the pregelation phase and prevents a indicated that a longer curing time is needed to slow and ordered chain growth.35 reach a similar depth of cure and to create optimal According to manufacturer information, Filtek performance for resin composite materials when Z250 contains filler particles that range in size using an LED light unit rather than a conventional from 0.01 to 3.5 μm. Most activation light sources tungsten halogen light unit.29,30 that are commercially available have a peak in the For a given irradiation time, it can be anticipat- range 450–500 nm. Research has shown that light ed that the microhardness of the composite de- scattering in the resin composite is maximal when creases as the thickness of resin cured increases. the filler particle size is half the wavelength of the In this study the LED light gave better performance activating light, resulting in a lower transmission than the high power halogen light with respect to coefficient and smaller depth of cure. The trans- hardness and depth of cure. In samples cured with mission coefficient is influenced by the wavelength the LED (TPB) light unit, the 1 mm and 2 mm levels of the light, the refractive indices of the resin and attained hardness values greater than or equal to fillers, and the nature and amount of the filler 80% of the corresponding surface hardness val- particles.36 In comparison with the LED light, ues. However, in the samples fast cured with the the EMS light has a much greater light intensity, QTH (EMS) light unit, only the 1 mm levels attained which actually increases light scattering and light 80% of the surface hardness values. The wave- attenuation, resulting in less camphorquinone ac- length range of the LED light used in this study tivation and resin conversion. This may also partly was 440–480 nm, which explains why greater de- explain the decreased DOC in the samples cured grees of hardness and conversion were achieved with the EMS light in this study. by this LED light with a 40 s irradiation than by the The light intensity and exposure time are QTH light with either a 4 s or 6 s irradiation. known to be the most important factors in tem- The halogen curing light produced significantly perature change.37 Moreover, the temperature lower depth of cure (DOC) and hardness values rise is known to increases with the power density than the LED curing light. Although the QTH light of the LED or QTH unit, but yielding a greater rise unit delivered a much greater power density, sug- for a given power density when using a QTH unit, gesting a higher degree of polymerization, the as corroborated by the findings of this study.15 LED light benefitted from a much longer expo- Another study indicated that photocuring blue sure time. The QTH light unit was only partly able light sources increase the temperature in tooth to compensate for this difference, increasing the tissue during in situ polymerization of resin com- DOC but not the hardness of the resin composite. posite and that a higher power density QTH light This result may be due to the reduced photoacti- source (Swiss Master Light) caused a greater in- vation time used by the EMS unit, representing a crease in tooth temperature than a high power lower amount of energy31 and a shorter period of LED light. The temperature rise was greater with time for light to penetrate deeper into the mate- increased exposure time, as found in this study as rial, since part of the light necessary for polym- well.38 erization is absorbed and scattered by the resin In this study, increasing the period of irradia- composite that has already been polymerized.32 tion with the QTH light from 4 s to 6 s only affected Another related factor may be that, although the DOC of the resin composite and the hardness much emitted light can satisfy the camphorqui- of its 2 mm level. This can be partly attributed to a none (CQ) absorption curve and initiate a polym- temperature rise by from 1.88°C to 3°C while cur- erization reaction, the highest probability of light ing the resin composite. absorption corresponds with the peak at 465 nm. The thermal variations that occur during the While the output of the halogen curing light has a photoactivation of composite resins are related broad spectrum, a great portion lies outside the both to the exothermic polymerization of the ma- CQ absorption curve, so 80% of the energy from terials and to the heat output from the dental cur- the halogen lamp is outside the useful curing ing light units. A cured composite, such as the range.7,8,33,34 cured composite disc in this study, is capable of 302 European Journal of Dentistry