Relationship between microstructure of the skin surface and surface reflection based on geometric optics

Abstract Background The behavior of reflected light in skin affects skin appearance and provides clues as to the internal condition of the skin. Surface topography is one of the central physical factors contributing to surface reflection. Objective We tried to clarify the relationship between microstructure of the skin surface and surface reflection based on geometric optics. Methods Microstructures and surface reflections in the left cheeks of adult females were evaluated. Skin topography was acquired measuring replicas using confocal laser microscopy. Surface topography was used to calculate arithmetical mean deviation of the surface ( S a ), and geometric index from gradient of the surface ( S grad ), which is expected to correlate with the directionality of surface reflection (DoSR) based on geometric optics. A surface reflection image was acquired from differently polarized pictures of a face, and the index of surface reflection ( I obs ) was calculated as the average pixel value of the area of shine. Correlations between indices were then evaluated. Results S grad and S a showed significant correlation ( p < 0.01) with I obs . However, S grad showed a higher correlation with the simulated surface reflection from the reflection model than S a . In addition, S grad can explain differences in DoSR for some panelists even in the case of an identical S a. Conclusions The topographic element involved in DoSR was extracted from height mapping. S grad reflects the ratio of flat area, offering a more effective indicator than S a for distinguishing topographic characteristics with respect to surface reflection.