Real-time transmitted irradiance as an indicator of polymerization efficiency in single-shade and bulk-fill resin composites

Real-time transmitted irradiance and its link to polymerization efficiency have rarely been assessed within a single experimental model. This in vitro study evaluated how material type and specimen thickness affect transmitted irradiance and microhardness in four bulk-fill composites (Omnichroma Bulk Flow, Charisma Bulk Flow ONE, SimpliShade Bulk Fill, and Filtek One Bulk Fill) and related the 20 s transmitted irradiance to the bottom-to-top microhardness ratio. Eighty specimens (10 per group) were prepared at 2 and 4 mm; transmitted irradiance was recorded at 1, 5, 10, 15, and 20 s and Vickers microhardness on the top and bottom surfaces. Transmitted irradiance differed significantly by material, thickness, and time ( p < 0.001), being highest for Omnichroma and lowest for SimpliShade, and higher at 2 than 4 mm. Filtek One showed the highest microhardness and Omnichroma the lowest. The bottom-to-top ratio exceeded 80% at 2 mm but not at 4 mm for any material. A significant overall correlation between transmitted irradiance and the microhardness ratio (Spearman’s ρ = 0.699, p < 0.001) was largely driven by thickness and non-significant within most subgroups. The association between transmitted irradiance and polymerization efficiency was largely attributable to specimen thickness and was not consistently observed within individual material–thickness groups. Therefore, transmitted irradiance should not be interpreted as an independent predictor of polymerization efficiency under the conditions evaluated. Real-time transmitted irradiance may provide a practical, non-destructive measure of light transmission behavior during photoactivation when comparing resin composites or specimen thicknesses. However, it should not be used alone to predict polymerization efficiency within a specific material and thickness.