Methods: A total of 180 disk-shaped samples were prepared from three nano-hybrid resin based composites: Grandio (VOCO), Tetric N-ceram (Ivoclar Vivadent), Simile (Pentron) in a teflon mold (diameter of 10 mm and height of 2 mm). 90 specimens for different distances were divided in three groups: (0 m, 3 mm, 9 mm). For each group, 10 specimens were made. Metal rings were used to control the distances. 90 specimens for different curing times were divided in three groups: (10 s, 20 s, 40 s). For each group 10 specimens were made. Vickers microhardness was measured on top and bottom surfaces. ANOVA test was used for statistical analysis.
Results: There was statistically significant difference in the microhardness between the groups with different times and distances (P<0.001). For all specimens top surface values were greater than bottom surfaces, but the ratio was different in three composites, ranging from 0.66 to 0.94. The highest top and bottom microhardness was measured for Grandio.
Conclusions: The results revealed that microhardness improves significantly by increasing the irradiation time or reducing the irradiation distance.