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

The explosive vaporization of melanosomes in situ in skin during pulsed laser irradiation (pulse duration <1-mu-s) 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, HO (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-degrees-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-degrees-C (SD, N = 3). The slope, partial H(O)/partial T(i), specifies the internal absorption coefficient, mu-a, within the melanosome: mu-a = -rho-C/(slope(1 + 7.1R(d))), where rho-C 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 (mu-a) for the melanosome interior (351-1064 nm) is presented based on H(O) 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.