What is selective photothermolysis?

The theory of selective photothermolysis assumes the laser light will pass through tissue until it targets a specific chromophore with an absorption spectrum corresponding to the wavelength of the laser. The target then absorbs the light, generating heat in the target tissue. The spread of heat is determined by the thermal relaxation time (TRT) of the tissue. This is the amount of time necessary for 50% of the peak heat to diffuse out of the target. It is important for the laser pulse duration not to exceed the thermal relaxation time of the target or the heat will diffuse into surrounding tissue, causing damage and possibly scarring. Thus, the TRT depends on the actual size of the target. The smaller the target, the shorter the TRT, and thus the need for shorter laser pulse durations.

Anderson RR, Parrish JA: Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation, Science 220:524–527, 1983.