We present in vivo optical properties of skin tissue assessed using spatially resolved diffuse reflectance spectroscopy. Data was collected in a sub-study of the Swedish CArdioPulmonary bioImage Study (SCAPIS). Measurements were performed on 3,809 subjects, 50-65 years, on the volar side of the forearm with the PeriFlux 6000 EPOS system.
The analysis consisted of an inverse Monte Carlo method where modeled spectra were non-linearly fitted to measured diffuse reflectance spectra at 0.4 and 1.2-mm source-detector fiber separations, respectively, between 475 and 850 nm. The model consisted of one epidermis layer with adaptable thickness and absorption caused by melanin, and two dermis layers. The upper dermis had a fixed thickness of 0.2 mm and the lower an infinite thickness. The absorption in the dermis layers depended on varying amounts of hemoglobin and its oxygen saturation. The reduced scattering coefficient, with three adaptable parameters, was equal in all layers in the model.
Absorption coefficient in epidermis times epidermis thickness, reflecting the total amount of melanin, was 0.19±0.16 [-] at 570 nm with a significant difference between winter (0.12±0.10) and summer (0.26±0.19). Melanin absorption implies an average of 3.9% tissue fraction of melanosomes in epidermis. Absorption coefficient at the 570 nm isosbestic point was 0.19±0.11 mm-1 in the upper dermis layer and 0.10±0.05 mm-1 in the lower. The reduced scattering coefficient was 2.3±0.5 mm-1 at 570 nm. Average sampling depth for all wavelengths and both separations was 0.43±0.03 mm.
