Fig. 5

Schematic diagram of the optical principle of the reflectometer. Arrows show the incident and reflected light beams. The top diagram (Principle) represents the highest reflection intensity. The light shines at a ~23° angle onto the enamel/tooth surface and it reflects back also at a ~23° angle (specular reflection) for the highest surface reflection intensity (SRI). The other diagrams represent originally sound teeth (S) and teeth that originally presented ETW (E). Before acid challenges, S_initial has clear undulations of the perikymata that scatter some of the light, while E_initial has a flat smoother surface that reflects most of the light. So, in S_initial a smaller proportion of the reflected beams will be captured by the detector (lower SRI), while in E_initial a greater proportion of the light will be reflected back to the detector (higher SRI). After the erosive challenges (S_24min and E_24min), the enamel surface is rougher, leading to more light scattering and, thus, lower SRI on both types of enamel (S and E)