Optical radiation transfer in the human skin and applications in in vivo remittance spectroscopy

Summary

Transmittance spectra (from ultraviolet (UV) to near infrared) and remittance (reflectance) spectra of human dermis and epidermis in vitro were analyzed using simple radiation transfer models. Fresh skin from operations and autopsy specimens was separated into layers by various techniques. Absorption spectra for melanins (synthetic, natural, and in situ) were determined, and the importance of the pigmented epidermal basal layer in Caucasian skin as a barrier to penetration of ultraviolet radiation into the dermis was documented. Previous studies of epidermal transmittance in Caucasian skin have failed to examine the basal layer, thus overestimating the fraction of incident UV radiation reaching the dermis. The transfer of 320–1000 nm radiation within washed dermis in vitro was shown to be primarily determined by optical scattering, not by absorption. This is because normal dermis lacks large quantities of pigments other than blood which absorb these wavelengths. Dermal scattering coefficients varied inversely with wavelength. These data and models can therefore be used to quantitate changes in both the amounts and depth profiles of pigments within the skin in vivo by interpreting in vivo remittance spectra. In vivo remittance spectra of erythematous skin induced by UV exposures were determined and interpreted in this fashion.