THE MELANOSOME: THRESHOLD TEMPERATURE FOR EXPLOSIVE VAPORIZATION AND INTERNAL ABSORPTION COEFFICIENT DURING PULSED LASER IRRADIATION

Abstract— —The explosive vaporization of melanosomes in situ in skin during pulsed laser irradiation (pulse duration <1 μs) is observed as a visible whitening of the superficial epidermal layer due to stratum corneum disruption. In this study, the ruby laser (694 nm) was used to determine the threshold radiant exposure, H₀ (J/cm²), required to elicit whitening for in vitro black (Negroid) human skin samples which were pre‐equilibrated at an initial temperature, T_i, of 0, 20, or 50°C. A plot of H₀ vs T_i yields a straight line whose x‐intercept indicates the threshold temperature of explosive vaporization to be 112 ± 7°C (SD, N = 3). The slope, ∂H ₀/∂T _i, specifies the internal absorption coefficient, μ_a, within the melanosome: μ_a = −ρC/(slope(1 + 7.1R _d)), where ρC is the product of density and specific heat, and R_d is the total diffuse reflectance from the skin. A summary of the absorption spectrum (μ_a for the melanosome interior (351–1064 nm) is presented based on H₀ data from this study and the literature. The in vivo absorption spectrum (380–820 nm) for human epidermal melanin was measured by an optical fiber spectrophotometer and is compared with the melanosome spectrum. 1991 American Society for Photobiology