Abstract: Objective　To evaluate the influence of thickness on the translucency parameters and light transmittance of Bulk-fill composites. Methods　Two Bulk-fill composite (Tetric N-Ceram Bulk Fill and Filtek Bulk-fill Flowable) and two traditional light-cured resin composite (Filtek Z100) were tested in this study. Each composite with different thickness (1、2、3 and 4mm) were prepared (n= 5 per group) and then, the L*, a* and b* values of each sample were analyzed with a spectrophotometer, which was used to calculate the translucency parameter (TP) with different thickness on black and white background. Furthermore, the light transmittance (T) of each resin at different thickness was measured. The results of each test were analyzed with one-way ANOVA followed by Tukey's HSD test using SPSS 21.0 software package. Results　For the four resins composites studied in this experiment, there is a trend of decreasing translucency parameter and light transmittance as the thickness increased. For the same thickness, the translucency parameters and light transmittance of Tetric N-ceram Bulk Fill and Filtek Bulk-fill Flowable were significantly higher than that of Filtek Z100 and Filtek Z250 (P<0.05). Conclusions　The transparency parameter and light transmittance of Tetric N-ceram Bulk Fill and Filtek Bulk-fill Flowable decreased with increasing thickness, but were significantly higher than that of Filtek Z100 and Filtek Z250.

Key words: Bulk fill composite, thickness, translucency, light transmittance, light cure