How Laser Light Meets Skin
Why a wavelength finds one structure and spares the rest.
A laser does not treat an area. It treats whatever inside that area happens to absorb the colour of light being delivered, for as long as the pulse lasts. Get the wavelength, the pulse duration and the cooling right and one structure is destroyed while its neighbours are untouched. Get them wrong and the same device burns. Everything else about laser treatment, including the reason the room is licensed, follows from that.
Reviewed by Dr Shahe Boghossian, GMC 5204600, Medical Consultant. Last reviewed 12 Aug 2026.
Schematic, not to scale. Longer wavelengths travel further before they are absorbed, which is why depth of target and choice of wavelength are the same decision.
Four settings. One of them is the difference between result and injury.
A practitioner does not choose a treatment so much as choose four numbers. The device offers ranges. The skin in front of them narrows those ranges to a much smaller window, and the window moves with skin tone, hair colour, body site, medication and recent sun exposure.
Wavelength
Every tissue component absorbs some colours of light and ignores others. Melanin, haemoglobin and water each have their own absorption curve. Wavelength decides which of them the energy lands on, and how deep it travels before it gets there.
Measured in nanometres
Pulse duration
Heat spreads. Deliver the energy faster than the target can shed it and the damage stays inside the target. Deliver it slowly and the heat leaks into surrounding tissue before the pulse has finished. This is the variable that separates a treatment from a burn.
Milliseconds, or less
Fluence
The energy delivered per unit of skin. Too little and the target survives, which is why an underpowered course produces nothing but expense. Too much and the surrounding tissue takes damage it did not need to take.
Joules per square centimetre
Surface cooling
The epidermis sits between the device and the target and contains melanin of its own, so it absorbs energy on the way through. Cooling the surface before, during and after the pulse keeps it below the threshold at which it blisters.
Contact, cryogen or air
Selectivity is a matter of timing.
In 1983 Anderson and Parrish showed that brief pulses of selectively absorbed light could damage one pigmented structure in living tissue and leave its neighbours intact. Their point was that precision does not have to come from aiming. It comes from the optical and thermal properties of the target itself.
The timing part is thermal relaxation time: how long a structure takes to shed roughly half the heat it has absorbed. It scales with the square of the target's diameter, so a target twice the size holds its heat four times as long. A melanosome, around a micrometre across, cools almost instantly. A hair follicle, some two hundred times wider, holds heat for tens of milliseconds.
That difference is what a practitioner is exploiting. Choose a pulse shorter than the target's relaxation time and the heat is trapped where it was absorbed. Choose one longer and it conducts outward into skin that was never the target, which is the mechanism behind most laser burns and most post-inflammatory pigment change.
None of this can be judged by eye during a treatment, which is why the settings are written into a treatment protocol beforehand rather than adjusted on instinct.
1983
The year selective photothermolysis was described, the principle every aesthetic laser still runs on.
Anderson & Parrish, Science, 1983
×4
Doubling a target's diameter quadruples the time it holds heat, because thermal relaxation time scales with the square of size.
Thermal relaxation, first principles
Which wavelength goes after what.
The clinic runs two light platforms. The Cynosure Elite+ is a Class 4 laser carrying two wavelengths, and the Lumecca is an intense pulsed light system. Between them they cover surface pigment and redness, hair at a range of skin tones, and selected superficial vessels.
| Wavelength | What absorbs it | How deep it reaches | LRC treatments |
|---|---|---|---|
| 515 to 590nm Filtered intense pulsed light | Oxyhaemoglobin and surface melanin | Epidermis and upper dermis | |
| 755nm Alexandrite | Melanin, strongly absorbed | Mid dermis, the depth of the follicle bulb | |
| 1064nm Nd:YAG | Melanin weakly, haemoglobin usefully | Deepest of the three |
A Class 4 beam does not stop at the couch.
Class 4 is the highest laser hazard class. It describes a device capable of causing injury from the direct beam, from a beam reflected off a surface, and from one scattered by a surface that looks matt to the eye. The hazard is therefore a property of the room, not of the handpiece.
The eye is the reason. Between roughly 400 and 1400 nanometres the eye transmits light to the retina and its optics focus it, concentrating the energy far above the level at which it entered. Wavelengths in the near infrared, the 1064nm used in hair removal among them, are invisible and produce no blink reflex, so the first sign of exposure can be the injury.
This is why wavelength-rated eyewear is worn by everyone present rather than by the patient alone, why reflective surfaces are covered, why the windows carry approved blinds, and why the treatment room is entered under control while the key is in the device.
In Nottinghamshire those controls are not left to the clinic's own judgement. They are licence conditions, set out under Part IV of the Nottinghamshire County Council Act 1985 and checked by inspection of the premises before a licence is granted.
Read what the licence requires, and the questions to ask any clinic
From the licensing guidance
"Class 4 lasers and intense pulsed light sources are powerful devices, which if faulty or incorrectly used can have the potential to cause serious injury to those operating them and the recipients of the treatment."
Guidance notes for establishments undertaking laser and intense pulsed light treatments, Nottinghamshire County Council Act 1985, Part IV.
Start here
The right wavelength is decided at consultation, not at the till.
Skin tone, hair colour, medication and recent sun exposure all change which settings are safe for you. Every laser and IPL course here begins with an assessment and a patch test before any full treatment is given.
