In clinical dental practice, adhesive defects between tooth substance and composite (Composite: type of synthetic resins which is used in dentistry as adhesives.) restorations [interfacial (Interface: boundary between adjacent substances or phases.) defects], which adversely affect restoration quality, cannot be avoided due to polymerization shrinkage, different thermal expansion of tooth substance and composite, and composite degradation.1 A longitudinal assessment of this interface would therefore be beneficial for revealing degradation effects in the bonding interface over time. During mastication, saliva and other liquids may enter the gaps under pressure, resulting in detachment of the adhesive interface, or deformation of restoration margins.2 This in turn may allow further penetration of saliva and microorganisms into the tooth—composite interface, bringing discoloration, hypersensitivity, plaque accumulation, and may potentially lead to the development of a caries lesion, which is the main reason for composite restorations to be replaced.3 However, this costly procedure can be delayed as long as possible with early detection and monitoring of gap or defect progression adjacent to restorations. The detection and assessment of interfacial gaps together with longitudinal monitoring of gap progression are therefore of great clinical significance. However, it is difficult to visually detect or assess gaps at the tooth—composite interface before large defects occur. Another important focus of assessment is to differentiate between merely discolored gaps4 on the one hand and defects with additional active carious damage on the other.