In short. Selective photothermolysis in plain language: chromophores, wavelength, pulse duration, and why the same laser behaves differently on different skin.
Key points
- Pick a wavelength the target absorbs, deliver it in a pulse shorter than the target needs to cool, use enough energy to reach the threshold effect, and cool the surface so it survives.
- Everything else — device choice, settings, test spots, intervals — is applied arithmetic on those four ideas.
The word laser suggests a single technology. In practice it is a light source with three adjustable properties, and almost everything about how a treatment feels, works, and risks injury follows from those three numbers.
Light is absorbed by colour
Laser light is absorbed by pigments called chromophores. In skin the important ones are melanin, which gives skin and hair their colour; haemoglobin, which colours blood and therefore vessels; water; and the pigment in tattoo ink. A wavelength is chosen because one of these absorbs it better than its neighbours do. That preference is the whole basis of laser medicine, and it is why one machine treats brown spots while another treats red ones.
Selective photothermolysis
The principle is called selective photothermolysis, and it means this: light of a chosen wavelength is absorbed preferentially by the target, converted to heat, and confined there long enough to damage it while leaving surrounding skin intact. Three conditions must hold: the wavelength must be absorbed by the target more than by its surroundings; the pulse must be short enough that heat cannot escape into neighbouring tissue; and there must be enough energy to reach the temperature that produces the intended effect. When the target and the surrounding skin absorb similarly — the situation in darker skin, or in a tan — selectivity falls and the margin for error narrows sharply.
Why pulse duration matters as much as power
A small target such as a fine vessel or a pigment granule cools very quickly; a large one such as a hair follicle cools slowly. Pulse duration is therefore matched to the size of the target. Too long, and heat leaks away before the target is damaged. Too short, and the energy arrives so fast that pressure and heat damage the tissue around the target, which is how purpura, blistering, and scarring begin.
Cooling, spot size, and depth
Skin is cooled before, during, or after the pulse to protect the surface while deeper structures are heated. Spot size affects how deeply light penetrates, because a larger spot loses less energy at its edges. Depth also decides what is treatable at all: epidermal pigment responds readily, while deep dermal pigment and deep vessels need longer wavelengths and accept more sessions for less complete clearance.
The plain-language summary
Pick a wavelength the target absorbs, deliver it in a pulse shorter than the target needs to cool, use enough energy to reach the threshold effect, and cool the surface so it survives. Everything else — device choice, settings, test spots, intervals — is applied arithmetic on those four ideas.
What this means for you
Two people with the same complaint may need different wavelengths, pulse durations, and numbers of sessions, because their skin colour and the depth and size of the target differ. A clinic proposing an identical protocol to everyone is applying it to the machine rather than to the skin.
Reasons to stop and ask
- Nobody has explained which target in your skin is being treated, or why that wavelength was chosen for it.
- You are told a single session will clear a concern that lies deep in the dermis, where light rarely reaches effectively.
- No patch test is offered even though you have a darker skin type, a recent tan, or a history of pigmentation after injury.
Frequently asked questions
What is selective photothermolysis in plain terms?
Light of a chosen wavelength is absorbed preferentially by the target, converted to heat, and confined there long enough to damage it while leaving surrounding skin intact. It requires the target to absorb more than its surroundings, a pulse shorter than the target needs to cool, and enough energy.
Why does pulse duration matter as much as power?
Because pulse duration is matched to the size of the target, which decides how fast it cools. Too long and heat leaks away before the target is damaged; too short and energy arrives so fast that pressure and heat damage the tissue around it.
Why can the same laser behave differently on different skin?
Because selectivity depends on how differently the target and its surroundings absorb the light. In darker skin, or in a tan, melanin absorbs across many wavelengths, so the margin between a therapeutic dose and an injurious one narrows sharply and settings must be adjusted accordingly.
Risks & Limitations
- Burns and blistering if settings do not match the skin.
- Post-inflammatory hyperpigmentation, particularly in darker skin types, which may take months to settle.
- Hypopigmentation (lightening) of treated skin, which can be permanent.
- Scarring, including hypertrophic scarring, which is uncommon but real.
- Infection, including reactivation of herpes simplex after resurfacing.
- Acne or milia flare during recovery.
- Results are incremental and most concerns need several sessions. Melasma may rebound or worsen with aggressive treatment.
When to Seek Care
- Blistering, weeping or broken skin beyond the expected reaction.
- Severe pain that is worsening rather than settling.
- Spreading redness, warmth, pus or fever.
- New darkening or firmness that looks like scarring at the treated area.
- Any change in a treated pigmented lesion — it should be reviewed, not retreated.
References & Further Reading
- American Academy of Dermatology (AAD) — laser and energy device information
- American Society for Laser Medicine & Surgery (ASLMS)
- National Medical Products Administration (NMPA, China) — device registration status
- Cochrane Library — systematic reviews in laser dermatology
This page is patient education, not medical advice, and it does not cite individual studies. Discuss your own case with a qualified clinician. To arrange an assessment at Chongqing West District Hospital, see Contact & Consultation.