Every pigment consult ends at the same fork. Either the melanin sits in the epidermis, a fraction of a millimetre down, or it's parked in the dermis below. Everything downstream changes with that one call: wavelength, dose, session count, the honesty of the forecast you give the patient. Fire before you've made it and you're guessing with a Class 4 laser.
This article is the working method, not the platform spec. The machine-level detail lives on our pages for nevus of Ota, melasma and freckle removal. What follows is the reading you do before any of them applies: what colour tells you, what history tells you, what a Wood's lamp adds, and where each of those tools lies to you. One caveat up front. This is technical background for equipment operators and buyers, not medical advice, and depth reading is not diagnosis. Anything that looks atypical goes to a dermatologist before it goes under a handpiece.
Why depth outranks everything on the spec sheet
The physics is short. Our engineering training material defines penetration depth as the distance over which light attenuates to 37% of its entry intensity, and that distance grows with wavelength. Green 532 nm light is stripped out close to the surface. Infrared 1064 nm keeps going and reaches the dermis. That's the entire mechanism connecting a depth estimate to a wavelength dial.
Selective photothermolysis — Anderson and Parrish, Science, 1983 — supplies the other half: match the wavelength to the target's absorption, deliver it faster than the target sheds heat, and injury stays confined. But the principle assumes you know where the target is. A perfectly selective pulse at the wrong depth is still a miss, and in skin a miss isn't free.
Depth also writes the prognosis. Epidermal pigment turns over with the epidermis, so clearance is quick — often very few sessions. Dermal pigment has to be fractured and hauled away by macrophages, which means a course measured in months to years. Mixed pigment, melasma above all, may never fully clear and can rebound. Quote sessions before reading depth and you've priced a job you haven't scoped.
Reading depth with your eyes
Colour is a crude depth gauge
Brown reads shallow. Blue, slate and grey read deep. The reason is scattering: shorter wavelengths bounce back out of the dermal tissue overlying deep melanin, so a dermal deposit takes on a blue-grey cast, while epidermal melanin shows its own brown or near-black colour honestly.
Our internal disease atlas is consistent with this across its case files. Freckles: yellowish-brown, 1–2 mm, epidermal. Melasma: tawny patches. Nevus of Ota: blue, brown to black — dermal melanocytosis. Acquired symmetrical zygomatic macules (often called Hori's naevus): grey-brown to black-grey, and the atlas traces them to melanocytes that stalled in the dermis during embryonic migration. Slate tones keep pointing down.
History does half the work
Ask three questions before touching a lamp. When did it appear? Does it change with the seasons? Who else in the family has it?
Sun-tracking is the sharpest discriminator. Freckles darken every summer and fade every winter — behaviour our atlas ties directly to their epidermal position. Nevus of Ota and the zygomatic macules show no relation to sun or season at all. A patient who tells you her spots ignore summer has already told you something a spec sheet can't.
Onset age separates the rest. Freckles arrive in childhood. Ota is roughly half congenital, half surfacing in adolescence. The zygomatic macules typically appear between the late teens and around forty, overwhelmingly in women. Melasma tracks pregnancy, contraceptive use and sun exposure. None of this is diagnostic on its own. Together it usually narrows the field to one or two candidates.
Pattern and morphology
The classic confusion is Hori-type macules versus melasma, because both sit symmetrically over the cheekbones in adult women. Look at the grain. The macules are discrete — well-demarcated spots of a few millimetres, a dozen or two per side per our atlas. Melasma is confluent: blotchy patches that merge. One is a dermal lesion that behaves like Ota under a laser. The other is the most treatment-resistant mixed pigment in the clinic. Mixing them up means either overtreating melasma or telling a dermal-pigment patient that a cream will fix her.
The Wood's lamp pass
A Wood's lamp is long-wave UVA around 365 nm, used in a darkened room at close range. It's the cheapest depth instrument you will ever own, and the logic behind it was published by Gilchrest and colleagues in the British Journal of Dermatology back in 1977: under Wood's light, contrast increases when the excess melanin is epidermal and falls away when it's dermal.
So the read is simple. Lesion pops harder against surrounding skin — epidermal. Lesion softens or vanishes — dermal. Patchy, with some areas enhancing and some not — mixed, which is the standard Wood's-lamp classification of melasma into epidermal, dermal and mixed types. Give your eyes a minute to adapt, kill the ambient light, and sweep the whole face. The lamp routinely finds satellite pigment the patient hasn't noticed.
What the lamp can't do
Three limits. First, it's a contrast trick, and contrast needs background: in deeply pigmented skin the surrounding melanin is so absorbing that the enhancement effect washes out, which makes readings on darker phototypes unreliable — exactly the patients where a wrong wavelength call costs most. Second, the lamp reports direction, not millimetres. It says shallower or deeper. It does not say how much of each. Third, it disagrees with other instruments often enough to stay humble: a 2022 study in the Indian Dermatology Online Journal comparing Wood's lamp against dermoscopy for melasma depth found only moderate agreement between the two.
Which is why our melasma protocol treats the test patch as the real tiebreaker. Our atlas specifies a trial area of 0.5 to 1 square centimetre, then a wait of several days, because some patients darken visibly after a pass and no lamp predicts which ones. The lamp shapes the plan. Tissue response confirms it.
Three depth buckets, three equipment answers
Epidermal: freckles, lentigines, sun spots
Shallow targets want the wavelength that dies shallow. That's 532 nm Q-switched, or broadband IPL for diffuse photodamage. Clearance is fast and the machine spec is modest — the full argument, including why fluence is derived from spot size rather than dialled, is on the freckle removal page.
Dermal: Ota, Ito, Hori-type macules
Nothing topical reaches the dermis, and 532 nm doesn't either. These lesions are 1064 nm Q-switched work, full stop, over a course that runs a year or more. Hardware matters here in a way it doesn't for freckles: our archive flatly advises against treating nevus of Ota with any unit whose single-pulse output sits below 200 mJ, listing four consequences — no significant effect, a longer course, raised scarring risk, more pain. Platform selection between our two Q-switched builds is covered on the nevus of Ota page and the QE-01 spec sheet.
Mixed: melasma
The hard bucket. Pigment in the epidermis, the dermis, or both at once — our atlas puts the histopathology in exactly those words — and published imaging work keeps finding a dermal component even in cases the lamp called epidermal. Treat it as the most conservative case on your list: large spot, low fluence at 1064 nm, mandatory test patch, and a patient briefed on relapse before session one. The melasma page carries the full protocol and the honest QE-01 versus QN-03 comparison.
What a wrong call costs
Run the failure modes. Fire 532 nm at a dermal lesion and the energy is absorbed high in tissue that was never the target: you injure the epidermis, leave the lesion untouched, and hand a pigment-prone patient a fresh insult — the mechanics of that outcome live on our post-inflammatory hyperpigmentation page. Push aggressive fluence into melasma you assumed was a simple sun spot and you can trigger the darkening the test patch exists to catch. Buy an underpowered bargain unit for what turns out to be an Ota-heavy caseload and you've bought the four problems listed above at a discount.
Notice that two of those three failures were purchasing errors before they were treatment errors. Depth reading isn't only chairside technique. It's how you scope which machine your caseload actually needs before the deposit leaves your account.
When you don't fire at all
Every lesion in our atlas shares a profile: flat, stable, symptom-free, following a known pattern. Anything outside that profile exits the laser conversation. A pigmented lesion that has recently changed, has an irregular or shifting border, is raised, bleeds, or simply looks different from its neighbours gets a dermatology referral and, where indicated, a biopsy — before any light source touches it. A Q-switched pass over an unrecognised melanoma doesn't just fail. It can delay the one diagnosis that matters.
Cheap insurance, all of it. A history that takes two minutes, a lamp that costs less than a handpiece cable, a test patch that costs a week. Spend those first. The wavelength decision then mostly makes itself — and the machine pages above tell you what to buy once it has.
Frequently asked questions
Can a Wood's lamp tell me exactly how deep the pigment sits?
No. It reports direction, not depth in millimetres: epidermal pigment gains contrast under the lamp, dermal pigment loses it. Published comparisons against dermoscopy show only moderate agreement, and readings blur on darker phototypes. Treat the lamp as one input alongside history and colour, and let a small test patch confirm the plan on tissue.
Why does dermal pigment look blue-grey instead of brown?
Scattering. Shorter wavelengths bounce back out of the tissue lying over a deep melanin deposit, so the lesion takes on a blue or slate cast. Epidermal melanin, sitting near the surface, shows its own brown to near-black colour. That's why the slate-grey tones of nevus of Ota or Hori-type macules point downward before you've switched anything on.
A lesion shows patchy enhancement under the lamp — some areas pop, some don't. What is it?
That patchy read is the classic mixed pattern, and in a symmetric facial distribution it usually means melasma with both epidermal and dermal components. It's the most conservative case on the list: large spot, low fluence, a mandatory test patch, and realistic expectations, as laid out on our melasma solution page.
Do freckles ever need a 1064 nm laser?
Ephelides themselves don't — they're epidermal, and 532 nm or IPL clears them quickly. The value of 1064 nm on the same console is the rest of the caseload: the dermal and mixed lesions that walk in the same door. That's the practical argument for a dual-wavelength Q-switched platform over a single-purpose device.
References
- Gilchrest BA, Fitzpatrick TB, Anderson RR, Parrish JA. Localization of melanin pigmentation in the skin with Wood's lamp. British Journal of Dermatology. 1977;96(3):245-248.
- Navya A, Pai V. Comparison of Dermoscope and Woods Lamp as a Tool to Study Melanin Depth in Melasma. Indian Dermatology Online Journal. 2022;13(3):366-369.
- Wood lamp skin examination. DermNet NZ.
- Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science. 1983;220(4596):524-527.