Objective To investigate the color stability and opacity of seven resin-based composite products after ultraviolet irradiation. Methods Seven kinds of light-cured resin-based composite products（Aelite Flo, Filtek Z350 Flowable, Clearfil AP-X, Dyract AP, Z100, Durafill VS, Filtek Z350 Universal） were tested in the study（five for each group）. Changes in color and opacity of test samples were determined before and after 8, 16 and 24 h ultraviolet irradiation using the CIE 1976 L*a*b* system and CIE 1931 XYZ system by a colorimeter. Results The color of resin disks suffering ultraviolet irradiation was different in differdnt irradiating time. Color difference values ranged from 1.85-3.21 for 8 h ultraviolet irradiation to 2.88-4.55 for 24 h ultraviolet irradiation. Opacity values ranged from 0.75-0.82 before ultraviolet irradiation to 0.80-0.98 after 24 h ultraviolet irradiation. Among all the groups, Z100 and Filtek Z350 Flowable showed the biggest color change, while Filtek Z350 Universal showed the highest opacity. Conclusion It can be concluded that for some products, ultraviolet irradiation leads to color instability and opacity variation.
