Effect of pigmentation density upon 2.0 microm laser irradiation thermal response

Yucatan mini-pigs with predominantly dark skin have been used to determine skin safety standards for infrared (IR) wavelength irradiation due to its anatomical similarity to all human skin. It has generally been argued that water is the principle absorber in the IR-B band and melanin has relatively low absorbance. To accept dark pigmented damage thresholds for skin with various melanin densities, it is necessary to investigate the potential role of melanin in producing skin injury as characterized by an erythermal response. A Yucatan mini-pig covered with lightly pigmented pink and darkly pigmented brown skin was used in this study. The significance of skin pigmentation was investigated by comparing the transient thermal response, absorption coefficient, and the threshold damage of instant redness within 1 min and persistent redness at 48 h post exposure for dark and light skin areas at 2.0 microm wavelength. The density of melanin granules did not significantly alter the thermal and optical properties of in vivo skin exposed to 2.0 microm laser irradiation. For Gaussian shaped beam radiation at 1 s exposure duration and 4.83 mm 1/e spot diameter, the average radiant exposures at instant and persistent redness thresholds were 3.88 J cm and 5.08 J cm for dark skin, respectively, as well as 4.09 J cm and 4.09 J cm for light colored skin. Subjectively speaking, however, lightly pigmented mini-pig skin was more suitable for damage threshold estimation because of the increased contrast for visual determination of redness on light skin.