Objectives: The aim of this study was to evaluate the degree of conversion and Vickers surface hardness of resin cement under a simulated ceramic restoration with three different curing units; a conventional halogen, a high intensity halogen and a light emitting diode system. Methods: A conventional halogen curing unit (Hilux) (40s), a high intensity halogen curing unit used in conventional and ramp mode (Optilux 501) (10s, 20s), and a light emitting diode system (Elipar FreeLight) (20s, 40 s) were used in this study. The dual curing resin cement (Variolink II) was cured under a simulated ceramic restoration (diameter 5mm, height 2mm) and the degree of conversion and Vickers surface hardness was measured. For degree of conversion measurement, 10 specimens were prepared for each group. The absorbance peaks were recorded using the diffuse-reflection mode of Fourier Transformation Infrared spectroscopy. For Vickers surface hardness measurement, 10 specimens were prepared for each group. A load of 200 N was applied for 15 s, and three evaluations of each of the samples were performed. Results: Degree of conversion achieved with Optilux 501 (20 s) was significantly higher than those of Hilux, Optilux 501 (10 s), Elipar FreeLight (20 s) and Elipar FreeLight (40 s). For Vickers surface hardness measurement, Optilux 501 (20 s) produced the highest surface hardness value. No significant differences were found among the Hilux, Optilux 501 (10 s), Elipar FreeLight (20 s) and Elipar FreeLight (40 s). Conclusion: The high intensity halogen curing unit used in ramp mode (20s) produced harder resin cement surfaces than did the conventional halogen curing unit, high intensity halogen curing unit used in conventional mode (10s) and light emitting diode system (20s, 40s), when cured through a simulated ceramic restoration.

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