Unit 1 · Energy & selective photothermolysis
Why a laser can destroy one target and leave its neighbours untouched
Every energy-based device works by delivering controlled energy to a targeted tissue component and inducing a precise biological response. This unit establishes why a laser can be tuned to injure one structure selectively, and which three parameters must align simultaneously for that selectivity to hold.
Framing
Two ways to heat skin
Every energy-based aesthetic device achieves its clinical effect by delivering a controlled form of energy to a targeted tissue component. Whether that energy is coherent light (laser), broadband light (IPL), or an oscillating electrical field (radiofrequency), the goal is the same: a precise, controlled biological response — typically thermal — while minimising collateral injury to surrounding structures.
How energy interacts with tissue is not supplementary knowledge. It is the framework inside which every treatment parameter, indication, adverse event and device comparison makes sense.
Energy devices deliver heat to skin through one of two mechanisms. Photothermolysis uses light energy absorbed by a specific chromophore — a light-absorbing molecule — converting light to heat within that target. Dielectric heating, used by radiofrequency, generates heat via oscillating electrical current based on tissue impedance — a chromophore-independent mechanism. Both produce thermal injury, but at different depths, with different selectivity, and through different biophysical pathways.
Learn · Selective photothermolysis
The theory that makes selective targeting possible
The theory of selective photothermolysis (SP), proposed by Anderson and Parrish in 1983, is the cornerstone of modern laser medicine. It explains how a laser can destroy a specific target — a pigmented lesion, a blood vessel — without damaging the tissue around it, provided three conditions are met simultaneously.
| Parameter | What it requires | Why it matters clinically |
|---|---|---|
| Wavelength (λ) | Must be strongly absorbed by the target chromophore and poorly absorbed by surrounding structures. | Determines which chromophore — and therefore which target — is treated. A 532 nm KTP laser targets melanin and oxyhaemoglobin; a 1064 nm Nd:YAG targets deeper vessels with less melanin competition. |
| Pulse duration (τ) | Must be equal to or shorter than the target's thermal relaxation time (TRT) — the time for the target to lose half its heat to surrounding tissue. | Shorter pulses confine heat to the target. Longer pulses let heat diffuse, risking collateral damage. TRT varies with target size. |
| Fluence (F) | Energy delivered per unit area (J/cm²) must reach the target's thermal damage threshold. | Too low: subtherapeutic effect. Too high: heat diffusion beyond the target, blistering or scarring. |
The principle that allows a laser to destroy a specific tissue target without damaging the tissue around it is:
Select an option to commit. The reasoning appears afterwards.
Selective photothermolysis, described by Anderson and Parrish in 1983, explains how a laser destroys a specific target while sparing the tissue around it — provided wavelength, pulse duration and fluence are all matched to that target simultaneously.
This is why a single laser device is never universally correct. Change the target and at least one of the three parameters usually has to change with it.
Predict · Pulse duration and thermal relaxation time
Why a leg vein tolerates a slower pulse than a melanosome
Pulse duration must be equal to or shorter than the target's thermal relaxation time (TRT) — the time it takes the target to lose half its heat to the surrounding tissue. Small targets and large targets do not share the same TRT. Before reading on, predict why target size would change the pulse duration a laser needs.
A melanosome and a leg vein are both legitimate laser targets, yet one needs a pulse in microseconds and the other tolerates a pulse in milliseconds. Why would target size change the pulse duration required for selective destruction?
Hold your answer before you open this. The value is in having reasoned through the relationship first.
Thermal relaxation time is proportional to the square of the target's diameter. A small target such as a melanosome (0.5–1 µm) loses heat to its surroundings almost instantly, giving it a TRT of around 1 microsecond — the laser pulse must be shorter still to keep the heat confined. A large target such as a leg vein (~1 mm) takes far longer to lose heat — its TRT is around 100 milliseconds — so it can be treated with a correspondingly longer pulse without heat escaping into the surrounding tissue.
This is the practical reason device settings change so much between indications. A pulse duration correct for a port wine stain vessel (TRT ~10 ms) would be far too long for a melanosome and would diffuse heat well beyond it.
Picosecond lasers deliver pulses of roughly 10⁻¹² seconds — far below the TRT of a melanosome. At that pulse duration, the effect on pigment is predominantly photoacoustic rather than photothermal: the pulse generates a pressure wave that fragments the pigment rather than heating it. This is a more selective, less damaging mode of fragmentation for recalcitrant pigment and tattoo ink than nanosecond Q-switched delivery.
Thermal relaxation time (TRT) is best defined as:
Select an option to commit. The reasoning appears afterwards.
TRT is the time a heated target takes to dissipate half of its peak temperature into the surrounding tissue. It is a property of the target's size and thermal diffusivity, not a setting the practitioner chooses directly.
The practitioner's lever is pulse duration, which must be set at or below the target's TRT. Get that relationship wrong and heat diffuses past the intended target regardless of how correctly the wavelength was chosen.
Learn · Fluence
Reaching the damage threshold without exceeding it
Fluence — the energy delivered per unit area, measured in J/cm² — must be sufficient to raise the target to its thermal damage threshold. Too low and the treatment is subtherapeutic; too high and heat diffuses beyond the target, producing blistering or scarring.
Fluence is titrated against patient skin type, target depth and chromophore density, not set once for a device and left unchanged. The same nominal fluence can be therapeutic in one patient and excessive in another, depending on how much of that energy the target chromophore actually absorbs.
A laser is set to the correct wavelength for its target chromophore, but the pulse duration selected is longer than the target's thermal relaxation time. The most likely clinical consequence is:
Select an option to commit. The reasoning appears afterwards.
Wavelength selects the target; pulse duration decides whether the resulting heat stays there. A pulse longer than TRT lets heat diffuse into the surrounding tissue well before the pulse finishes delivering energy.
This is why the three parameters of selective photothermolysis have to be assessed together, not as a checklist ticked off one at a time. A correct wavelength paired with the wrong pulse duration still produces a treatment that heats the right structure and the wrong volume of tissue around it.
Unit 1 summary
Clinical takeaways
- Selective photothermolysis requires three conditions at once. Wavelength, pulse duration and fluence all have to be matched to the target simultaneously — meeting two of the three is not enough to avoid collateral damage.
- Wavelength decides the target; pulse duration decides where the heat stays. A wavelength strongly absorbed by the intended chromophore still allows heat to escape into surrounding tissue if the pulse outlasts the target's thermal relaxation time.
- Thermal relaxation time scales with target size. Small targets such as melanosomes relax in microseconds; large targets such as leg veins relax over tens to hundreds of milliseconds — which is why device settings change so much between indications.
- Fluence must clear the damage threshold without overshooting it. Too little energy is subtherapeutic; too much diffuses beyond the target and risks blistering or scarring, and the correct level depends on skin type, depth and chromophore density.