to be damaged by heat diffusion from the highly pigmented/strongly absorbing portion of the chromophore (the heater or absorber). Such nonspecific thermal damage evokes the concept of thermal damage time (TDT). The TDT of a target is the time required for irreversible target damage with sparing of the surrounding tissue. For a nonuniformly absorbing target structure, the TDT is the time it takes for the outermost part of the target to reach a target damage temperature through heat diffusion from the heated chromophore.
According to the concept of extended selective photothermolysis, target damage can still be selective even though the TDT is many times as long as the thermal relaxation time (TRT) of the actual target.
This new extended theory of selective thermal damage of nonuniformly pigmented structures in biological tissue postulates that the target is destroyed by heat diffusion from the absorbing chromophore to the target but not by direct heating from laser irradiation, as is seen with selective photothermolysis. This theory has now been applied to the treatment of unwanted hair. Ultimately, the use of hair removal lasers expanded rapidly with the subsequent development of appropriate cooling devices that minimized epidermal injury.
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