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Thermal relaxation time (TRT) is how long a heated target takes to lose half its heat. Keeping the laser pulse shorter than the TRT confines damage to the target—the key to safe, selective treatment.
Thermal relaxation time (TRT) is the time a heated structure needs to dissipate half of its absorbed heat. It scales with size: a small target (a melanosome) cools in nanoseconds, while a large one (a coarse terminal hair) takes milliseconds.
TRT is the bridge between theory and settings. To confine heat to the target—the heart of selective photothermolysis—the pulse must be delivered in less time than the target takes to cool. Fire faster than the TRT and the heat stays trapped where you want it; fire slower and heat leaks into surrounding tissue, raising the risk of scarring and pigment change. This is precisely why pigment lasers use nanosecond and picosecond pulses while hair lasers use milliseconds.
Understanding TRT is what lets a skilled operator translate ‘what am I treating?’ into the correct pulse duration on the device.
Every target has a cooling clock—fire faster than it ticks and the heat stays exactly where you aimed it.
Selective photothermolysis is the founding principle of modern laser therapy: by choosing
Pulse duration (pulse width) is how long each burst of laser energy lasts.
Q-switched describes a laser that releases its energy in extremely short, high-power nanos
Thermal relaxation time (TRT) is how long a heated target takes to lose half its heat—keeping the laser pulse shorter than the TRT confines damage to the target. A core concept behind safe, selective laser treatment.
