Abstract
The epidermal melanin content affects most dermatologic treatments involving light, and can limit the therapeutic success significantly. Therefore, knowledge of the optical properties of skin is required. This study investigates how the concentration of melanin influences visible reflectance spectra of skin and the relationship to threshold radiant energy fluence for melanosomal or melanocyte destruction. Reflectance spectra were measured at 28 pigmented human skin sites in vivo. For Asian and Caucasian subjects, measured reflectance values varied over the same range, while significantly lower values were recorded for African individuals. Epidermal melanin absorption coefficients measured at 694 nm were about 2500 m-1 for African, and 300–1200 m-16 for Caucasian and Asian skin. Twenty-five skin sites were exposed to ruby laser pulses (694 nm), where the pulse duration was long enough to allow heat diffusion between melanosomes. Hypopigmentation occurred, on average, at 12 and 26 J cm-2 for sun-exposed and sun-protected white skin, respectively, while slightly lower threshold values resulted from the measured spectra. As visible reflectance spectra reveal information regarding skin pigmentation and individual threshold doses for melanosomal damage, a use as a diagnostic tool in various dermatological laser treatments is apparent.
Highlights
Human skin colour varies significantly between individuals, dependent on race, sun exposure and age
Visible reflectance spectra were measured at the upper arm or forearm, and average melanin absorption coefficients were determined
The skin of the African females was not irradiated by the laser
Summary
Human skin colour varies significantly between individuals, dependent on race, sun exposure and age. A proper analysis of the reflectance spectrum of human skin in the visible wavelength region (380-780 nm) might, reveal important diagnostic information. Additional information exists in the near infra-red spectrum ranging from 780 to about 1500 nm wavelength. The optical penetration depth in the ultra-violet (u.v.) part of the spectrum is much less than the epidermal thickness. Little information can be extracted from reflectance spectra in this region. Normal skin colour originates in the presence of specific chromophores such as melanin, haemoglobin, bilirubin and carotene.
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