THE DYNAMICS OF LASER-INDUCED CHANGES IN HUMAN SKIN AUTOFLUORESCENCE - EXPERIMENTAL MEASUREMENTS AND THEORETICAL MODELING

To study the temporal dynamics of human skin autofluorescence photoble aching, we measured the autofluorescence spectral changes of skin in v ivo during continuous exposure to 442 nm (He-Cd) laser light. Integral intensities were calculated for various spectral wavelength bands and plotted as a function of time. Mathematical analysis of the time func tion revealed a double-exponential photobleaching process: I(t) = a ex p(-t/tau(1)) + b exp(-t/tau(2)) + c, in which tau(1) and tau(2) differ ed by an order of magnitude. A hypothesis for the mechanism of the dou ble-exponential photobleaching dynamics was proposed and evaluated usi ng Monte Carlo modeling of light propagation in the skin and autofluor escence escape from skin, By combining the fluorophore microdistributi ons, Monte Carlo simulation results and the variation in fluorescence decrease parameters (a, b, c, tau(1), tau(2)) with increasing exposure intensities a biophysical explanation for the double-exponential phot obleaching function was elucidated. The fast decrease term corresponds to laser-induced photobleaching in the stratum corneum, while the slo w decrease term represents fluorophore changes in the dermis, The meas ured autofluorescence photobleaching dynamics can be used to determine the fractional contributions of different skin layers to the total au tofluorescence signal measured in vivo.