Thermal relaxation time (TRT) is how long a heated target inside the skin needs to shed roughly half its heat. It is the single most useful number for choosing pulse duration. The rule that follows from it is simple: deliver your energy in a time at or below the target's TRT and the heat stays trapped in the target, damaging it while sparing the tissue around it. Deliver it too slowly and the heat leaks out before it does the job, so you either miss the target or burn the skin.
This is why a tattoo needs nanoseconds and a hair needs milliseconds. The two targets differ enormously in size, and TRT scales with size. Below is how the physics works and how to translate it into settings you can dial in on the machine.
What is thermal relaxation time?
Thermal relaxation time is the time it takes a light-absorbing target in the skin to cool down by about half after it has been heated. The concept comes from the theory of selective photothermolysis, described by Anderson and Parrish in Science in 1983, which remains the foundation of every pigment, vascular, and hair-removal laser sold today. Their central point: if a pulse is short compared with the target's TRT, the absorbed heat is confined to the target and cannot diffuse into neighbouring tissue during the pulse.
The HONKON training reference used across the Pmise device family frames the same idea in clinical terms: TRT is the time a target needs to release the bulk of its heat, and heat that stays confined is heat that does useful, selective work. Once you accept that, pulse duration stops being a guess and becomes a matching exercise.
Short pulse relative to TRT: heat is confined, damage is selective. Long pulse relative to TRT: heat diffuses out, selectivity is lost and collateral warming rises.
Why does TRT scale with target size?
Bigger targets cool more slowly, so they have a longer thermal relaxation time. As a working approximation, TRT is roughly proportional to the square of the target's diameter. Double the size of the target and its cooling time goes up by about a factor of four. This one relationship explains the entire pulse-duration ladder, from nanoseconds to hundreds of milliseconds.
Think about the three chromophores clinics treat most often, ordered by physical size:
- Tattoo ink particles and melanosomes: sub-micron to micron scale, so they cool almost instantly and have TRTs in the sub-microsecond range.
- Blood vessels: tens to a few hundred microns across, giving TRTs commonly cited in the millisecond range, longer for larger vessels.
- Hair follicles: the follicular unit is large and the heat has to spread from the pigmented shaft to the surrounding follicle, so the effective damage time sits in the tens of milliseconds.
The size-to-time ladder is why a single laser cannot do everything well. The pulse a Q-switched device fires would barely warm a hair follicle, and the pulse a hair laser fires would let a tattoo particle's heat drain away before the particle shatters.
The core rule: match pulse duration to the target
Set your pulse duration at or below the target's TRT for confinement, and no shorter than you need. This is the practical heart of selective heating. A pulse much shorter than the TRT still works, but it drives up peak power and can tip a thermal effect into a violent photomechanical one. A pulse longer than the TRT is where the trouble starts, because heat spreads out of the target and into the epidermis and dermis you wanted to protect.
There is a useful refinement here. For a hair follicle the true target that absorbs light (the pigmented matrix and shaft) is not the same as the structure you want to destroy (the follicle). The extended theory of selective photothermolysis, published by Altshuler, Anderson and colleagues in 2001, introduced the thermal damage time to cover this case: you sometimes want a pulse a little longer than the absorber's own TRT so heat has time to conduct out to the full target. That is exactly why hair removal favours longer millisecond pulses rather than the shortest one available.
Why tattoos and pigment need nanoseconds
Pigment particles are tiny, so their thermal relaxation time is extremely short and the pulse has to be shorter still. In the original selective-photothermolysis work, melanosomes were targeted with pulses of only tens of nanoseconds. Modern Q-switched Nd:YAG systems work in the single-digit-nanosecond regime for the same reason. A nanosecond pulse packs its energy into so little time that peak power is enormous, and the particle heats and fragments before its heat can escape into the surrounding skin.
This is a photomechanical, not a slow-cooking, effect. The absorbed energy shatters the particle so the body can clear the fragments. Pmise Q-switched platforms are built around this: the 1064QCH fires pulses in the single-nanosecond range, and the passive Q-switched MVEH pigment system uses comparable nanosecond pulse widths. See the Q-switched Nd:YAG laser for the pigment and tattoo configuration, and our comparison of EO Q-switched versus standard Nd:YAG if you are weighing pulse stability between switch types.
Why hair and vessels need milliseconds
Hair follicles and blood vessels are large, so their TRT is long and short pulses simply do not suit them. Fire a nanosecond pulse at a follicle and you get a small explosion at the pigment, not the sustained heating that coagulates the whole follicle. Instead you want a pulse in the millisecond range that heats the pigmented shaft and lets that heat conduct out to the growth structures.
The HONKON light-tissue reference gives a clean teaching example: a melanin-bearing follicle heated with a pulse on the order of 100 milliseconds undergoes progressive thermal coagulation, while the same fluence delivered to the epidermis would injure it in a slightly shorter time. That thin margin is why every hair and vascular laser needs strong epidermal cooling. Pmise diode systems such as the 808CH deliver 808 nm energy with adjustable pulse durations across the millisecond range (roughly 5 to 300 ms per the device manual), letting the operator lengthen the pulse for coarse hair and thicker skin. The long-pulse 1064B Nd:YAG covers vessels and darker skin types with millisecond pulses. See the diode laser for hair removal, and our breakdown of diode vs alexandrite vs Nd:YAG for hair removal.
Target, size, TRT and the pulse regime that fits
The table below lines up common targets with their approximate size, the commonly cited thermal relaxation range, and the pulse regime and device type that follow. Treat the TRT figures as order-of-magnitude guides drawn from selective-photothermolysis theory, not exact clinical constants.
| Target | Approx. size | Typical TRT range | Pulse regime | Device type |
|---|---|---|---|---|
| Tattoo ink / melanosome | Sub-micron | Sub-microsecond | Nanoseconds | Q-switched Nd:YAG |
| Epidermal pigment (freckle, lentigo) | Micron scale | Microseconds | Nanoseconds to microseconds | Q-switched / short-pulse |
| Small blood vessel | Tens of microns | Milliseconds | Milliseconds | Long-pulse Nd:YAG |
| Hair follicle | Hundreds of microns | Tens of milliseconds | Tens of milliseconds | Diode / long-pulse Nd:YAG |
How to set pulse duration in the clinic
Work from the target outward. Once you know what you are treating, its size fixes the TRT, and the TRT fixes your pulse window. A repeatable order of operations:
- Identify the target chromophore: ink, epidermal pigment, vessel, or hair.
- Estimate its size. Larger targets tolerate and often need longer pulses.
- Choose a pulse at or below the target's TRT. Shorter favours fragmentation; longer favours coagulation.
- For a large target whose absorber is smaller than the structure (like a follicle), allow a slightly longer pulse so heat conducts out to the full target.
- Set epidermal cooling appropriately, especially for millisecond hair and vascular work and for darker skin.
- Test on a small area, observe the immediate endpoint, and adjust before treating the full field.
A quick sanity checklist before you fire:
- Is the pulse short enough to confine heat in the target?
- Is it long enough to avoid an unwanted explosive effect on a large target?
- Is the epidermis protected by contact or spray cooling?
- Does the endpoint match the goal (fragmentation for pigment, coagulation for hair and vessels)?
Frequently asked questions
Is thermal relaxation time the same as pulse duration?
No. Thermal relaxation time is a property of the target, set by its size and how fast it sheds heat. Pulse duration is a setting you control on the device. The skill in laser treatment is matching the second to the first: choose a pulse duration at or below the target's TRT so the heat stays confined and the effect is selective.
Why can't one laser treat both tattoos and hair?
Because the targets differ in size by orders of magnitude, and so do their thermal relaxation times. Ink particles need nanosecond pulses to fragment before their heat escapes, while hair follicles need millisecond pulses that heat the whole structure. A device optimised for one regime delivers the wrong pulse for the other, which is why clinics run separate Q-switched and long-pulse or diode platforms.
What happens if my pulse is longer than the target's TRT?
Heat diffuses out of the target during the pulse and warms the surrounding tissue. You lose selectivity, the target may not reach its damage threshold, and the epidermis heats up more than you want, raising the risk of burns, blistering, or pigment change. If anything, err toward a pulse at or just under the TRT and rely on cooling for epidermal safety.
Does skin colour change the pulse duration I should use?
It changes your safety margin more than the target's TRT. Darker skin holds more epidermal melanin, which competes for the light, so operators often use longer wavelengths such as 1064 nm, robust cooling, and carefully chosen pulse durations to protect the surface. The target's thermal relaxation time is unchanged, but the room for error is smaller.
Written by the Pmise Technical Team. Pmise manufactures laser and light-based aesthetic systems including Q-switched and long-pulse Nd:YAG, diode, and IPL platforms, and supports clinics and distributors worldwide with application training grounded in laser-tissue physics.
