Unit 2 · Chromophores & laser classification
Why the same wavelength is a good choice in one patient and a risk in another
A laser wavelength is never selected in isolation. This unit works through the three chromophores that govern every aesthetic laser decision, then classifies the major laser systems by the target each one is built to reach — so that a device comparison becomes a question of chromophore and depth, not brand names.
Learn · Primary skin chromophores
Three molecules decide what a laser can reach
A chromophore is any molecule that absorbs light at a specific wavelength. In aesthetic laser medicine, three primary chromophores determine the targets available for treatment — melanin, oxyhaemoglobin, and water. Each has a distinct absorption spectrum, and wavelength selection has to account for the relative absorption of all three, not just the intended target.
- Melanin — peak absorption 300–800 nm (broad spectrum)
- Located in melanosomes within melanocytes and keratinocytes. Primary chromophore for pigmented lesions — solar lentigines, café-au-lait macules, epidermal melasma, naevi, and tattoo ink (an exogenous chromophore). Absorption is broad across the visible and near-UV spectrum and decreases at longer wavelengths.
- Oxyhaemoglobin — peak absorption 418 nm (Soret band), 542 nm, 577 nm
- Located within red blood cells in cutaneous vasculature. Primary chromophore for vascular lesions — telangiectasia, rosacea, port wine stains, haemangiomas, poikiloderma. Wavelengths selected for vascular targets exploit this absorption to heat the vessel wall, producing thermocoagulation and vessel collapse.
- Water — peak absorption 2940 nm (Er:YAG); 10,600 nm (CO₂)
- Constitutes 65–72% of dermal tissue and is the dominant chromophore in the infrared spectrum. Lasers targeting water heat intracellular and extracellular water to vaporisation, producing controlled ablation. Depth of ablation and residual thermal damage (RTD) determine both efficacy and recovery profile.
Eumelanin (brown/black) and phaeomelanin (red/yellow) have slightly different absorption spectra — relevant when treating red and yellow tattoo pigments, which do not respond identically to a wavelength chosen for black ink. Deoxyhaemoglobin also has a distinct spectrum from oxyhaemoglobin, relevant in venous lesion treatment where the vessel content is less oxygenated.
The primary chromophore targeted by a 532 nm laser used for vascular lesion treatment is:
Select an option to commit. The reasoning appears afterwards.
Oxyhaemoglobin has absorption peaks close to 532 nm, which is why this wavelength is used for vascular targets — the light is absorbed within red blood cells in the cutaneous vasculature, heating the vessel wall and producing thermocoagulation.
The same wavelength is also strongly absorbed by melanin, which is the source of its competing chromophore risk. A device selected for one chromophore is rarely selecting for only one chromophore in practice.
Learn · Laser classification by tissue target
Six systems, three chromophores, one selection logic
Lasers used in aesthetic medicine are classified by wavelength (which determines chromophore affinity), pulse characteristics, and delivery mode — ablative, non-ablative, or fractional. The table below summarises the major classes in clinical use.
| Device | Type & wavelength | Mechanism | Primary clinical applications | Key consideration |
|---|---|---|---|---|
| CO₂ laser | Ablative — water — 10,600 nm | Water absorption vaporises tissue cells. RTD of 20–150 µm drives collagen contraction and stimulates neosynthesis. | Full-face resurfacing, deep rhytids, acne scarring, skin laxity. | Longest recovery of the resurfacing lasers (7–14 days); requires antiviral prophylaxis. |
| Er:YAG laser | Ablative — water — 2940 nm | 12–18× greater water absorption than CO₂. Precise ablation with minimal RTD (~5–25 µm). | Superficial-to-moderate resurfacing, fine rhytids, faster-recovery patients. | Less haemostasis and collagen stimulation per session than CO₂. |
| Pulsed dye laser (PDL) | Non-ablative — oxyhaemoglobin — 585/595 nm | Pulse matched to vessel TRT (~0.45–6 ms). Thermocoagulation of the vessel wall. | Port wine stains, telangiectasia, facial erythema, rosacea. | Purpura at therapeutic fluences is an expected marker of adequate treatment. |
| KTP / Nd:YAG 532 nm | Non-ablative — oxyhaemoglobin & melanin — 532 nm | Frequency-doubled Nd:YAG. Strong absorption by both target chromophores; shallow penetration (~0.75 mm). | Superficial telangiectasia, rosacea, epidermal pigmented lesions. | High melanin competition — increased PIH risk in Fitzpatrick III and above. |
| Nd:YAG 1064 nm | Non-ablative — deep vascular & pigment — 1064 nm | Penetrates to 5–6 mm; lower melanin absorption than 532 nm. | Deep vascular lesions, laser hair removal and tattoo removal in darker skin, dermal pigment. | Safer in darker skin but less efficient at superficial pigment targets. |
| IPL (Intense Pulsed Light) | Non-laser broadband light — 500–1200 nm | Non-coherent, filtered broadband light. Selectivity achieved via cut-off filters, not monochromaticity. | Photorejuvenation, combined pigmented-and-vascular presentations. | Reduced selectivity versus laser — higher collateral risk in darker skin types. |
Predict · Coherence and selectivity
Why IPL is not simply "a weaker laser"
IPL is used for many of the same indications as vascular and pigment lasers, yet it is not classified as a laser at all. Before reading on, predict what structural difference between IPL and a true laser would make IPL less selective, not just less powerful.
A laser delivers monochromatic, collimated light at one precise wavelength. What does IPL deliver instead, and why does that difference reduce its selectivity rather than simply its power?
Hold your answer before you open this. The value is in having reasoned through the mechanism first.
IPL delivers non-coherent, broadband light filtered to a selected wavelength range — for example 515–1200 nm for pigment, or 560–1200 nm for vascular work — rather than a single wavelength. Selectivity is achieved with cut-off filters rather than true monochromaticity, so multiple chromophores are inevitably targeted at once.
This is why IPL is well suited to combined pigmented-and-vascular presentations, and also why it carries a higher risk of epidermal collateral damage in darker skin types than a single-wavelength laser tuned precisely to one chromophore.
Learn · Competing chromophore absorption
Why Fitzpatrick skin type changes the device decision
In clinical practice, competing chromophore absorption is a constant consideration. A wavelength selected to target melanin — 532 nm KTP, for example — also has significant oxyhaemoglobin absorption, potentially causing purpura in vascular skin or vessel rupture. The same wavelength is more strongly absorbed by epidermal melanin in darker skin types (Fitzpatrick IV–VI), increasing the risk of post-inflammatory hyperpigmentation (PIH) and epidermal damage.
Longer wavelengths such as 1064 nm Nd:YAG are relatively less absorbed by melanin, penetrate deeper, and are generally safer for darker skin — but less efficient at superficial pigment targets. Chromophore competition directly drives the clinical decision to use longer wavelengths, longer cooling times, lower fluences, or an alternate modality in higher Fitzpatrick phototypes.
Melanin does not become chromophore-neutral simply because the intended target is a vessel or a different pigment. In darker skin types, epidermal melanin competes for every wavelength below the near-infrared range, and settings chosen for a lighter phototype can produce epidermal injury in a darker one at identical fluence.
A colleague argues that IPL should simply be thought of as a lower-powered laser for the same indications. Based on the structural difference between the two technologies, the accurate correction is:
Select an option to commit. The reasoning appears afterwards.
IPL is non-coherent, broadband light, filtered to a selected range rather than tuned to one wavelength. Selectivity is achieved with cut-off filters, not with the monochromaticity and collimation that give a laser its precision.
The clinical consequence is that IPL can treat combined pigmented-and-vascular presentations well, but it carries a higher risk of epidermal collateral damage in darker skin types than a single-wavelength laser tuned to one chromophore.
Fitzpatrick skin type is a critical variable in laser treatment planning because:
Select an option to commit. The reasoning appears afterwards.
Melanin is present throughout the epidermis in greater density in higher Fitzpatrick phototypes, and it competes for absorption with almost every wavelength below the near-infrared range. A wavelength chosen to treat pigment or vessels also heats that competing epidermal melanin.
This is why the practical response is a wavelength and parameter decision — longer wavelengths, longer cooling, lower fluence, or an alternate modality — rather than a single rule applied uniformly across every device and every phototype.
Unit 2 summary
Clinical takeaways
- Three chromophores govern every laser decision. Melanin, oxyhaemoglobin and water each have a distinct absorption spectrum, and wavelength selection has to account for the relative absorption of all three, not just the intended target.
- Device classification follows chromophore and depth, not brand. Ablative devices target water for vaporisation; non-ablative vascular and pigment devices target oxyhaemoglobin and melanin at wavelengths chosen for absorption and penetration depth.
- IPL is a different technology, not a weaker laser. Non-coherent, filtered broadband light reduces selectivity rather than power, which is why it treats combined presentations well but carries more collateral risk in darker skin.
- Competing chromophore absorption drives phototype-based decisions. Melanin in darker skin competes with the intended target at almost every wavelength below the near-infrared range, which is why higher Fitzpatrick phototypes change the wavelength, cooling and fluence decision rather than simply the power setting.