Thermal relaxation time is how long a heated target inside the skin takes to shed roughly 63% of its heat. Know that number and pulse duration stops being a guess. Keep the pulse at or under the target's thermal relaxation time and heat stays where you put it. Let it run long and that heat drains into tissue you wanted to leave alone.
That's why a tattoo needs nanoseconds and a leg hair needs milliseconds. Same physics, very different targets.
What is thermal relaxation time?
Thermal relaxation time, or TRT, is the time a light-absorbing structure in the skin needs to release about 63% of the heat it just absorbed. Our internal training documentation states it in those terms. The concept grew out of selective photothermolysis, the framework Anderson and Parrish published in Science in 1983, which still underpins every pigment, vascular and hair device sold today.
Their argument was elegant. When a pulse is short relative to the target's TRT, absorbed energy has nowhere to go: it piles up inside the target and leaves neighbouring tissue cool. Stretch past the TRT and conduction wins, so you heat a blurry region instead of a defined target. Our primer on selective photothermolysis walks through the theory.
Pulse shorter than TRT: heat stays confined, damage stays selective. Pulse longer: heat spreads and the epidermis absorbs your mistakes, with one exception covered below.
Why does TRT scale with target size?
Big things cool slowly. Small things cool fast. Our technical archive puts it as a square relationship: TRT rises with the square of target size, so doubling the diameter multiplies cooling time by roughly four. That explains the whole pulse ladder, picoseconds to hundreds of milliseconds. The three chromophores clinics treat most, smallest first:
- Tattoo ink and melanosomes. Sub-micron to micron. They dump heat almost instantly, so TRT sits in the sub-microsecond range.
- Blood vessels. Tens to hundreds of microns. Cooling lands in milliseconds, bigger vessels at the slow end.
- Hair follicles. Heat travels from the pigmented shaft outward, pushing useful treatment time into tens of milliseconds.
The rule you use at the machine
Set your pulse at or below the target's TRT, and no shorter than the job needs. That's selective heating in one sentence. Much shorter still confines heat, but peak power climbs and a controlled thermal effect flips into a violent mechanical one. Longer is usually where burns come from.
Usually, not always. There's a wrinkle that matters most in hair removal. The structure absorbing the light is often not the structure you want to destroy. Our archive is specific: an operator aims at the isthmus of the follicle and the dermal papilla, yet the tissue soaking up the light is the hair matrix and shaft. The same documentation defines thermal damage time as the interval the whole target needs to cool by about 63%, absorber plus surrounding follicle. It runs longer than TRT, giving conducted heat time to reach the parts that matter.
Altshuler, Anderson and colleagues formalised this in their 2001 extended theory of selective photothermolysis in Lasers in Surgery and Medicine. The conclusion is blunt: for a non-uniformly pigmented target, the correct pulse is significantly longer than the target's own TRT. Hence long-pulse hair lasers.
Why tattoo ink and pigment need nanoseconds
Pigment particles are tiny, their TRT is brutally short, and your pulse has to be shorter still. Nanoseconds, not microseconds. The effect at that timescale is largely photomechanical: the particle absorbs, expands and fragments before much heat escapes, and the body clears the debris over the following weeks.
Our engineering archive illustrates what Q-switching buys you. A 200 mJ Nd:YAG pulse runs about 400 microseconds unswitched; Q-switched to 10 nanoseconds at the same energy, it is 1/40,000 as long, so peak power climbs 40,000-fold. The energy never changed. What it does changed completely. Our documentation notes Q-switched pigment work produces almost no thermal effect, coagulation or ablation.
Keep shrinking the target and the regime shifts again. Picosecond platforms, which DermNet NZ defines as systems with pulse durations under one nanosecond, push the interaction further from heat toward pressure. DermNet describes the injury as predominantly photoacoustic: pigment fragments while photothermal damage to surrounding tissue stays limited. DermNet also reports fewer sessions and fewer adverse effects, though cost and availability keep nanosecond systems in most treatment rooms. Neither regime touches hair or vessels; both are far too short. Our Q-switched versus picosecond comparison covers where each earns its place.
Pmise Q-switched platforms sit in that nanosecond regime. The Q-switched Nd:YAG laser range covers pigment and tattoo configurations, including the dual-wavelength Pmise QN-09. Choosing between switch types? Our long pulse versus Q-switched guide covers where each belongs on a menu.
Why hair and vessels need milliseconds
Follicles and vessels are big and their TRT is long, so a nanosecond pulse is the wrong tool. You get a bang at the pigment, not sustained heating.
Our archive carries a worked teaching example. Read it as arithmetic under one set of assumptions, not a clinical constant. At a single fixed fluence, with no epidermal cooling in the picture, a melanin-bearing follicle coagulates progressively under a 100 ms pulse, while that same fluence needs on the order of 99 ms to injure the epidermis. Change wavelength, spot size, fluence or skin type and both figures move independently. No device has a universal one-millisecond safety window. What survives is the shape of the problem: on a pigmented epidermis, the gap between follicle damage and skin damage can close alarmingly. So cooling is mandatory on millisecond platforms.
Starting parameters from our diode hair-removal protocol show where that lands. Arms, legs and bikini start in the 20 to 45 ms band; upper lip sits nearer 30 to 40 ms; for Fitzpatrick type V the protocol drops both fluence and the top of the pulse range, typically 4 to 5 J with durations around 20 to 35 ms. Coarse hair wants the longer end, darker skin a test patch. Our guide to Fitzpatrick skin types and laser settings goes deeper, and the diode laser for hair removal line shows the platforms these protocols run on.
The commercial endpoint follows the same logic. FDA 510(k) documentation for diode hair-removal devices defines permanent hair reduction as a stable, long-term reduction in regrowing hairs, measured at 6, 9 and 12 months after the course. That comes from coagulating follicles across several sessions.
Target size, TRT and pulse regime at a glance
Treat these as order-of-magnitude guides from theory, not clinical constants. Your device manual and your own test spots decide the final number.
| Target | Approx. size | TRT order of magnitude | Pulse regime | Device type |
|---|---|---|---|---|
| Fine or resistant ink | Sub-micron | Sub-microsecond | Picoseconds (under 1 ns) | Picosecond platform |
| Tattoo ink particle / melanosome | Sub-micron | Sub-microsecond | Nanoseconds | Q-switched Nd:YAG |
| Epidermal pigment (lentigo) | Micron scale | Microseconds | Nanoseconds | Q-switched Nd:YAG |
| Small blood vessel | Tens of microns | Milliseconds | Milliseconds | Long-pulse Nd:YAG |
| Hair follicle | Hundreds of microns | Tens of milliseconds | Tens of ms (thermal damage time) | Diode or long-pulse Nd:YAG |
How to set pulse duration, step by step
Work outward from the target. Identify it, size it, and the window follows:
- Name the chromophore you are treating: ink, epidermal pigment, vessel, or hair.
- Estimate its size. Larger targets need and tolerate longer pulses.
- Pick a pulse at or below its TRT. Shorter favours fragmentation, longer favours coagulation.
- If the absorber is smaller than the structure you want to destroy, as with a follicle, allow a longer pulse so conducted heat reaches the whole target.
- Match cooling to the pulse. Millisecond work on darker skin is where the margin is thinnest.
- Test a small area, read the endpoint, adjust, then treat the field.
Quick check before you press the pedal:
- Short enough to keep heat inside the target?
- Long enough to avoid an explosive effect on a large target?
- Epidermis protected, by contact or spray cooling?
- Endpoint matching intent, fragmentation for pigment or coagulation for hair and vessels?
- Pulse interval long enough to shed heat between shots?
That last one gets overlooked in a busy salon running back-to-back sessions. Our archive is clear: a wider pulse interval gives skin more time to shed heat, while pigment work wants short intervals and narrow pulse widths to spike target temperature.
Buying rather than operating? Send us the treatment list you plan to sell, skin types included. Our engineers will map it to the pulse-width ranges and cooling each item needs, then send full parameter sheets for the relevant Q-switched and diode platforms. Pmise handles pre-sales parameter configuration, OEM and ODM builds, export documentation, spare parts and operator training, so your team knows why a number was chosen. Talk to our technical team for those sheets and a configuration proposal for your menu.
Frequently Asked Questions
Is thermal relaxation time the same thing as pulse duration?
No. Thermal relaxation time belongs to the target, set by how big it is and how fast it sheds heat, and you can't change it. Pulse duration is a setting on your machine. The craft is matching the second to the first: choose a pulse at or under the target's TRT so heat stays confined and the effect stays selective.
Why can't one laser handle both tattoos and hair?
The two targets differ in size by orders of magnitude, so their thermal relaxation times do too. Ink particles need nanosecond or shorter pulses to fragment before their heat leaks away. Follicles need millisecond pulses that heat the whole structure by conduction. A device built for one regime delivers the wrong pulse for the other, so clinics run both.
What happens if my pulse is longer than the target's TRT?
It depends on whether the absorber and the structure you want to destroy are the same thing. With a uniformly pigmented target, an ink deposit, a melanosome or a small vessel, they are, so running past the TRT lets heat leak outward: selectivity drops, and blistering, crusting or pigment change can follow. When the absorber is smaller than the target structure, as with a hair follicle, a pulse longer than the absorber's TRT is correct under thermal damage time reasoning. The risk then sits with the epidermis, so cooling carries the safety.
Does skin colour change the pulse duration I should choose?
It changes your safety margin, not the target. Darker skin carries more epidermal melanin competing for the light, so operators favour deeper wavelengths such as 1064 nm, stronger cooling, and the durations in the manufacturer's protocol. The follicle's thermal relaxation time hasn't moved. Your room for error has.
Written by the Pmise Technical Team. Pmise builds laser and light-based aesthetic systems, including Q-switched and long-pulse Nd:YAG, diode and IPL platforms, and trains clinic and distributor staff in the physics behind every parameter on the panel.
