Laser and Skin

Lasers and skin: one principle behind many devices

Every laser used on skin, whether it removes a tattoo, closes a red vessel or resurfaces a wrinkle, works on one idea: selective photothermolysis, described in 1983, matches a wavelength to a single absorbing target in the skin (melanin, haemoglobin or water) and delivers it in a pulse shorter than that target needs to lose its heat. Change the wavelength and the target changes; change the pulse length from nanoseconds to milliseconds and a mechanical shattering effect turns into a thermal one. More than a dozen device names, from the 532 nanometre KTP laser to the 10,600 nanometre CO2 laser, are engineering answers to those same two questions.

One idea explains hundreds of laser names

Inside skin, three substances absorb light strongly enough to matter for a laser: melanin, the pigment in freckles, sun spots and hair roots; haemoglobin, the red pigment in blood; and water, which makes up roughly 70% of the dermis. Each absorbs a different part of the spectrum, so choosing a wavelength chooses a target. Deliver that wavelength in a pulse shorter than the time the target needs to lose half its heat to the tissue around it, and the target heats up while its neighbours stay largely unaffected. That single rule, selective photothermolysis, explains almost every laser sold for skin treatment today.

The idea has a specific birthday. In 1983, R. Rox Anderson and John Parrish at Massachusetts General Hospital and Harvard Medical School published the concept in the journal Science, demonstrating selective damage to blood vessels with a 577 nanometre pulse lasting 300 nanoseconds, and to melanin-containing structures with a 351 nanometre pulse lasting only 20 nanoseconds. Every laser platform launched since, whatever its brand name, is an engineering answer to the two questions that 1983 paper asked: which wavelength reaches the target, and how short must the pulse be to stay selective.

Ablative, non-ablative and fractional are three separate choices

Ablative describes a laser that vaporises the outer layer of skin outright. Carbon dioxide lasers work at 10,600 nanometres and erbium:YAG lasers at 2,940 nanometres, both tuned to water, and both remove the epidermis in a controlled pass. Healing over the following 1 to 2 weeks builds fresh skin and new collagen, which is why ablative resurfacing is quoted at around 50% improvement in wrinkles and scars, alongside the longest recovery of any laser category.

Non-ablative lasers use the same wavelengths, or nearby ones, at settings that heat the dermis while a cooled tip keeps the surface intact. Nothing visibly sheds, so downtime is short, but the effect of any single session is milder, and a course of 4 to 6 sessions, spaced about 4 weeks apart, is typical rather than one strong pass.

Fractional is not a third device, it is a pattern. Instead of treating a whole surface, the laser fires into a grid of columns a fraction of a millimetre wide, some systems reaching up to 210,000 spots in a single pass, and leaves untouched skin standing between them. Those islands act as a reservoir of healthy cells, which is why a fractional treatment heals faster than the same energy delivered as one continuous surface, whether the laser itself is ablative or non-ablative.

Pulse duration turns the same energy into two different effects

Wavelength chooses the target, pulse duration chooses what happens to it. Pulses in the nanosecond range, roughly 10 to 100 nanoseconds, and picosecond pulses under 1,000 picoseconds arrive faster than a small particle of pigment or tattoo ink can shed its heat, so the particle shatters mechanically rather than simply warming up. That mechanical effect, not a burn, is why Q-switched and picosecond lasers are the standard choice for tattoo ink and stubborn pigment.

Pulses in the millisecond range, typically 5 to 100 milliseconds, are long enough for heat to spread from the target into the tissue immediately around it, a thermal rather than a mechanical effect. Hair removal depends on exactly that spread: the laser targets melanin in the hair shaft, but the heat has to travel a short distance to damage the follicle, so a longer, thermal pulse works better than a short, mechanical one.

Each structure in the skin has its own thermal relaxation time, the time it needs to lose half its absorbed heat. A single melanosome loses it in under 1 microsecond, a hair follicle takes milliseconds, a blood vessel several hundred microns wide takes longer still. Match the pulse to that number and the surrounding skin is spared; miss it and the treatment either fails to heat the target or damages everything around it.

Cooling and skin type set the safety margin

Any energy the target fails to absorb ends up in the surrounding skin as heat, which is what cooling is designed to remove. Cold air, a chilled contact window, or a burst of cryogen spray applied before, during or after the pulse pulls heat out of the epidermis and lowers the chance of a burn. A device running at meaningful power with no cooling of any kind is worth questioning.

Melanin competes with every other target for the same light, so more of it in the skin means more energy absorbed where it should not be. On the Fitzpatrick scale of skin types, from type I (pale, always burns) to type VI (deeply pigmented, never burns), types IV to VI carry a higher risk of burns and of pigment shifting darker or lighter after treatment, particularly with the shorter wavelengths used against melanin. Clinics adjust for this with longer wavelengths such as the 1,064 nanometre Nd:YAG, longer pulses and lower fluence, and a test patch on a small area is worth the extra 20 to 30 minutes it costs before a full session.

A few conditions change what is safe to treat, whatever the skin type. Isotretinoin thins the skin and slows healing, so most clinics wait 6 to 12 months after a course before an ablative or fractional pass. An active infection or a cold sore in the treatment area is reason to postpone, since the healing that follows an ablative pass can reactivate herpes simplex. Blood thinners raise the chance of bruising after a vascular laser, and most clinics defer elective treatment during pregnancy as a precaution. A short medical history taken before the first session, not after, is what catches these.

Tanned skin shifts every phototype a step darker for the purposes of laser safety, which is why sun and sunbeds are avoided for roughly 4 weeks before and after a session on any device tuned to melanin. Ask which skin type a clinic has assessed you as and which settings follow from it; an answer that stays the same for every client is a default setting, not an assessment. See the Skin Type Finder for a first read on your own phototype before you book.

Training decides who should hold the device

The devices that carry the most risk, ablative resurfacing, vascular lasers, and anything used on pigmented lesions or to remove tattoos, are also the ones most often reserved for a trained physician. Regulators differ in how they draw that line: Germany, for one example among several, has restricted these specific applications to physicians with additional training since 31 December 2020, while the United States classifies medical lasers as prescription devices under FDA oversight, to be used under a licensed practitioner's supervision. The detail of the rule changes by country; the reason behind it, that a burn or a pigment shift is easier to prevent than to fix, does not.

Laser hair removal sits in a different category in many places, open to trained non-physician operators, but dermatology bodies still recommend a board certified dermatologist for anything beyond routine hair removal and warn specifically against non-medical settings for pigmented lesions, vascular work or tattoo removal, citing limited training and equipment as the main risk. Ask which qualification the operator holds, which device and wavelength will be used on your skin type, and whether a physician is on site or on call; a confident, specific answer to all three questions is the baseline, not a bonus. For hair specifically, permanent hair removal covers what to expect session by session.

Questions worth asking before any laser session

Frequently asked

What does selective photothermolysis mean?

It means a wavelength of light is absorbed by one specific target in the skin, melanin, haemoglobin or water, and heats that target while the surrounding tissue stays largely unaffected. The idea was first described in 1983 and underlies every laser used on skin today.

What is the difference between ablative and non-ablative lasers?

Ablative lasers, such as CO2 at 10,600 nanometres or erbium:YAG at 2,940 nanometres, vaporise the surface of the skin and need 1 to 2 weeks of healing. Non-ablative lasers heat the tissue underneath while a cooled tip keeps the surface intact, which means far less downtime but usually more sessions for the same result.

What does fractional mean on a laser?

Fractional describes a pattern, not a device type: energy is delivered in thousands of microscopic columns, sometimes up to 210,000 in one pass, leaving untreated islands of skin between them. Those islands speed healing, and fractional versions exist of both ablative and non-ablative lasers.

Why does pulse duration matter as much as wavelength?

Wavelength picks the target, but pulse duration decides how it reacts. Pulses under 1,000 picoseconds shatter small particles such as pigment or tattoo ink mechanically, while pulses of 5 to 100 milliseconds heat larger structures such as a hair follicle thermally.

Is laser treatment safe for darker skin?

Yes, when wavelength, pulse length and energy are adjusted for it. Skin types IV to VI on the Fitzpatrick scale absorb more energy in the epidermis itself, so clinics use longer wavelengths such as the 1,064 nanometre Nd:YAG, longer pulses and lower fluence, plus a test patch first.

Why is cooling part of almost every laser session?

Because any energy the target does not absorb ends up as heat in the surrounding skin. Cold air, a chilled contact window or a cryogen spray removes that extra heat during the pulse and is one of the main reasons burns are rare when a trained operator uses the right settings.

Sources

• DermNet NZ: Lasers in dermatology · wavelengths by laser type, the three chromophores (melanin, haemoglobin, water), ablative versus non-ablative, safety requirements and adverse effects
• American Academy of Dermatology: Skin conditions lasers can treat · which conditions lasers treat, typical session counts, board certified dermatologist advice, higher burn and pigment risk in darker skin, warning against non-medical settings
• Martella A, Raichi M, Photoepilation and skin photorejuvenation: an update, Dermatology Reports 2017 (PMC) · IPL wavelength range 590 to 1,200 nm, diode versus IPL hair reduction at 3 and 12 months, Fitzpatrick skin type guidance, cooling filters, ruby laser hair removal approval history
• Anderson RR, Parrish JA, Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation, Science 1983 · the original 1983 paper naming selective photothermolysis; the publisher answers 403 to an automated fetch (a known Science/AAAS access block, not a broken link), abstract figures (577 nm at 300 ns on vessels, 351 nm at 20 ns on melanosomes) confirmed independently

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This guide is for general information. It does not replace medical or professional cosmetic advice or an examination.