Wood's Lamp: What Your Skin Reveals Under UV Light
The Wood's lamp is one of the oldest, simplest, and most underappreciated tools in dermatology and skin assessment. It looks unremarkable — a handheld ultraviolet light used in a darkened room — but in trained hands it reveals a layer of diagnostic information that is completely invisible under normal light. Pigment disorders, fungal infections, bacterial infections, and the depth of melanin in the skin all announce themselves through characteristic colours of fluorescence. Knowing how to read those colours is the difference between guessing at a skin lesion and narrowing it to a working diagnosis in seconds.
This guide explains what the Wood's lamp is, the physics of how it works, what each fluorescence colour actually means, and how it is used across pigment disorders, infections, and skin assessment — written from the perspective of a skin professional who uses UV assessment as part of a thorough clinical examination.
What does a Wood's lamp show? A Wood's lamp emits long-wave ultraviolet light (UV-A, 320–400nm) that causes certain skin substances and microorganisms to fluoresce in characteristic colours. Depigmented skin (vitiligo) glows bright blue-white; fungal infections like tinea versicolor fluoresce yellow-green to coppery-orange; the bacterial infection erythrasma glows coral-red; Pseudomonas fluoresces green; and epidermal pigmentation (like melasma) becomes more sharply defined while dermal pigment does not. Each colour points toward a specific diagnosis, making the Wood's lamp a rapid, non-invasive first-line assessment tool.1,2,3
What Is a Wood's Lamp?
The Wood's lamp — also called Wood's light, a black light, or a UV lamp — was developed by the American physicist Robert Williams Wood in 1903. It consists of a high-pressure mercury arc lamp fitted with a Wood's filter (made of barium silicate with nickel oxide) that blocks visible light and transmits only long-wave ultraviolet radiation, peaking around 365nm.3,4
This wavelength sits just beyond the violet end of the visible spectrum — invisible to the human eye, which is why the room must be dark for the examination. When this UV-A light strikes the skin, certain molecules absorb the energy and re-emit it as visible light of a longer wavelength. That re-emission is fluorescence — and the specific colour depends entirely on which substance is doing the absorbing.3
- UV-A emission: The lamp emits long-wave ultraviolet light (320–400nm, peak ~365nm), invisible to the human eye.3
- Absorption by fluorophores: Substances in the skin or produced by microorganisms — called fluorophores — absorb the UV energy. Examples include melanin, collagen, bacterial porphyrins, and fungal metabolites.3,4
- Re-emission as visible light: The absorbed energy is re-emitted at a longer, now-visible wavelength — producing the characteristic fluorescent colour seen in the darkened room.3
- Melanin as a filter: Melanin absorbs UV light. Where melanin is absent (vitiligo), UV penetrates and underlying fluorophores glow brightly; where melanin is increased, the light is absorbed and the area appears darker with sharper borders.2,5
What Each Colour Means: The Wood's Lamp Fluorescence Guide
This is the heart of Wood's lamp interpretation. Each characteristic colour points toward a category of diagnosis.
| Fluorescence Colour | Indicates | Cause of Fluorescence |
|---|---|---|
| Bright blue-white | Vitiligo / depigmentation | Absence of melanin allows UV penetration; accumulated biopterins fluoresce |
| Yellow-green to coppery-orange | Tinea/pityriasis versicolor (Malassezia) | Porphyrin pityrialactone produced by the yeast |
| Coral-red / coral-pink | Erythrasma (Corynebacterium) | Coproporphyrin III produced by the bacterium |
| Green / blue-green | Pseudomonas infection (wounds, burns) | Pyoverdin/pyocyanin pigments from the bacterium |
| Blue-green to green-yellow | Tinea capitis (Microsporum species) | Pteridine / tryptophan metabolites in infected hairs |
| Orange-red | Progressive macular hypomelanosis (C. acnes) | Porphyrins produced by Cutibacterium acnes in follicles |
| Pale yellow | Trichomycosis axillaris (Corynebacterium) | Bacterial concretions on hair shafts |
| Enhanced brown (sharper) | Epidermal pigmentation (melasma, PIH) | Increased epidermal melanin absorbs UV — contrast heightened |
| Unchanged (no accentuation) | Dermal pigmentation | Deeper dermal melanin not accentuated by UV — key differentiator |
| Pink-red | Porphyria (urine/skin) | Excess porphyrins in tissue and urine |
Pigment Disorders Under the Wood's Lamp
Pigment assessment is one of the most valuable applications of the Wood's lamp — and one of the most relevant to aesthetic and functional skin practice.
Vitiligo and Depigmentation
Because melanin normally absorbs UV light, its complete absence in vitiligo allows the UV to penetrate and causes the depigmented patches to fluoresce a striking bright blue-white, with sharply demarcated borders. This is far more visible than under normal light — the Wood's lamp is particularly valuable for detecting early or subtle vitiligo in fair-skinned individuals where the contrast under normal light is minimal, and for accurately mapping the true extent of depigmentation before treatment.1,2
Melasma: Epidermal vs Dermal — The Critical Distinction
This is one of the most clinically useful things the Wood's lamp does. In hyperpigmentation such as melasma or post-inflammatory pigmentation, the lamp distinguishes where the pigment sits:
- Epidermal pigment — becomes more pronounced and sharply defined under the Wood's lamp, because the increased surface melanin strongly absorbs UV, heightening contrast.
- Dermal pigment — appears unchanged under the Wood's lamp, because the deeper melanin is not accentuated by the UV light.
This distinction is not academic — it directly determines treatment. Epidermal melasma responds well to topical agents and superficial resurfacing, while dermal melasma is far more resistant and requires a different, more cautious approach. Assessing pigment depth before treatment is a mark of proper clinical practice.2,5
Hypopigmentation Disorders
The Wood's lamp helps distinguish true depigmentation (vitiligo — bright blue-white) from hypopigmentation (partial pigment loss). Conditions like nevus depigmentosus and the ash-leaf macules of tuberous sclerosis show enhanced but less dramatic contrast, aiding differentiation from vitiligo.1
Fungal Infections Under the Wood's Lamp
Tinea versicolor, caused primarily by Malassezia globosa, produces hypo- or hyperpigmented scaly patches on the chest and back. Under the Wood's lamp, active lesions fluoresce yellow-green to coppery-orange due to the porphyrin pityrialactone produced by the yeast. This is the same Malassezia genus involved in seborrheic dermatitis, making the Wood's lamp a useful tool for assessing the extent of Malassezia-driven skin conditions.1,3
Certain scalp fungal infections fluoresce under the Wood's lamp: Microsporum species (M. canis, M. audouinii) produce a blue-green fluorescence in infected hair shafts due to pteridine, and M. ferrugineum produces a green-yellow fluorescence from tryptophan metabolites. Importantly, not all tinea capitis fluoresces — Trichophyton species (a common cause) generally do not fluoresce, so a negative Wood's lamp does not exclude fungal scalp infection. This is a key limitation to understand.1,4
The Wood's lamp has limited utility for diagnosing onychomycosis (nail fungal infection), because the most common causative dermatophytes, yeasts, and moulds do not reliably fluoresce.1
Bacterial Infections Under the Wood's Lamp
Erythrasma — Coral-Red
Erythrasma, caused by Corynebacterium minutissimum, presents as reddish-brown patches in skin folds (groin, armpits, between toes) that are easily mistaken for fungal infection. Under the Wood's lamp it fluoresces a distinctive coral-red (coral-pink) due to coproporphyrin III produced by the bacteria — an immediate and reliable differentiator from tinea. This is one of the most classic and useful Wood's lamp findings.1,2
Pseudomonas — Green
Pseudomonas aeruginosa infection — in wounds, burns, or the nail — fluoresces green under the Wood's lamp due to the pigment pyoverdin. This can provide rapid bedside indication of Pseudomonas colonisation before culture results are available.2
Progressive Macular Hypomelanosis — Orange-Red
Caused by Cutibacterium acnes (the same organism involved in acne), progressive macular hypomelanosis mimics tinea versicolor and post-inflammatory hypopigmentation. Under the Wood's lamp, C. acnes fluoresces orange-red within the pilosebaceous follicles — clinching a diagnosis that is otherwise easily confused.1
Wood's Lamp in Aesthetic Skin Assessment
Beyond diagnosing infections and pigment disorders, the Wood's lamp is a valuable tool in aesthetic and functional skin assessment — evaluating the skin's overall condition below what is visible under normal light:
- Pigmentation depth mapping: Determining whether pigmentation is epidermal or dermal before selecting treatment — the single most useful aesthetic application.
- Sun damage: UV-induced pigment irregularities and photodamage become more visible, revealing the true extent of accumulated sun damage.
- Oiliness and dehydration: Sebum and areas of dryness can show characteristic patterns, aiding skin-type assessment.
- Bacterial and fungal contributors: Identifying C. acnes porphyrin fluorescence (orange-red follicular dots) in acne assessment, or Malassezia involvement in seborrheic presentations.
A thorough skin assessment goes beyond what is visible under normal light — using tools like Wood's lamp examination and dermoscopy to evaluate pigment depth, microbial contributors, and the underlying condition of the skin, informing a treatment plan matched to what is actually happening beneath the surface.
Limitations and False Results
The Wood's lamp is a screening and assessment aid — not a definitive diagnostic test on its own. Understanding its limitations is part of using it well:
- False negatives from washing: Recent washing removes the fluorescing porphyrins and metabolites, producing a false-negative. The area should not be cleaned before examination.6
- False positives from contaminants: Deodorants, soaps, cosmetics, topical medications, lint, and even some fabrics fluoresce and can mislead. Petrolatum fluoresces; tetracycline and certain other substances do too.3,6
- Not all organisms fluoresce: Trichophyton tinea capitis and most causes of onychomycosis do not fluoresce — a negative result does not exclude infection.1,4
- Requires proper conditions: Inadequate room darkness, insufficient lamp warm-up, or a non-dark-adapted examiner all reduce accuracy.3
What colour is vitiligo under a Wood's lamp?
Vitiligo fluoresces a bright blue-white (sometimes described as chalk-white or milky-white) under a Wood's lamp, with sharply demarcated borders. This occurs because vitiligo involves the complete loss of melanocytes and therefore melanin — and since melanin normally absorbs UV light, its absence allows the UV to penetrate freely and the accumulated biopterins in the depigmented skin to fluoresce brightly. The contrast is far more striking than under normal light, which makes the Wood's lamp especially valuable for detecting early vitiligo in fair skin and for accurately mapping the true borders of depigmentation before and during treatment.1,2,5
Can a Wood's lamp diagnose skin conditions on its own?
No — the Wood's lamp is a valuable assessment aid that narrows the diagnosis, but it does not replace a full clinical examination or confirmatory testing. Its fluorescence findings are highly suggestive — coral-red for erythrasma, blue-white for vitiligo, yellow-green for tinea versicolor — but false negatives (from recent washing or non-fluorescing organisms) and false positives (from cosmetics, soaps, and contaminants) mean the findings must be interpreted in clinical context. It is best understood as a rapid, non-invasive tool that guides the next diagnostic step, whether that is microscopy, culture, dermoscopy, or a treatment decision.3,6
Frequently Asked Questions: Wood's Lamp
Is a Wood's lamp examination safe?
Yes — a Wood's lamp examination is safe, non-invasive, and painless. It uses long-wave UV-A light at low intensity for a brief examination, posing no meaningful risk to skin or eyes during normal use. As a sensible precaution, the examiner avoids shining the light directly into the eyes. There is no recovery time and no side effects. It is suitable for patients of all ages.3
Why does the room need to be dark for a Wood's lamp exam?
The fluorescence produced by skin substances and microorganisms is relatively faint, and the UV light itself is invisible. In a lit room, ambient light overwhelms the subtle fluorescent colours, making them impossible to see. A dark room — ideally windowless — combined with a dark-adapted examiner (whose eyes have adjusted to the darkness) allows the characteristic colours to be perceived clearly against the surrounding skin.3,6
Should I prepare my skin before a Wood's lamp examination?
The most important preparation is to not wash or apply products to the area beforehand. Washing can remove the fluorescing porphyrins and fungal metabolites that produce diagnostic colours, causing a false-negative result. Deodorants, moisturisers, makeup, perfumes, and sunscreen can all fluoresce and cause misleading readings, so the skin should be clean of applied products but not freshly washed immediately before the exam.3,6
What is the difference between a Wood's lamp and dermoscopy?
They are complementary tools that reveal different things. A Wood's lamp uses ultraviolet light to detect fluorescence — revealing pigment depth, fungal and bacterial infections, and depigmentation across a broad area. Dermoscopy uses magnification and polarised light to examine the subsurface structures of individual lesions — pigment networks, vessels, and patterns used primarily in assessing moles and skin cancer risk. A thorough skin assessment may use both: the Wood's lamp for broad pigment and infection screening, dermoscopy for detailed lesion analysis.1
Can a Wood's lamp detect skin cancer?
The Wood's lamp is not a skin cancer detection tool in itself — that role belongs to dermoscopy and clinical examination. However, it has a supporting role: it can help delineate the borders of certain lesions (such as lentigo maligna) before treatment, and it is used to visualise fluorescence in some photodynamic therapy protocols where lesions are treated with fluorescent precursor agents. For skin cancer screening specifically, dermoscopy and full clinical examination are the appropriate tools.3,4
A comprehensive skin assessment uses proper tools — Wood's lamp, dermoscopy, and functional evaluation — to understand what is actually happening in your skin before recommending treatment.
Further Reading & Trusted Sources
- Revealing the Unseen: A Review of Wood's Lamp in Dermatology — Journal of Clinical and Aesthetic Dermatology (JCAD).
- Wood's Light — StatPearls, NCBI Bookshelf — open access, NIH.
- Wood Lamp Skin Examination — DermNet NZ
- Wood's Lamp Examination — Cleveland Clinic
References
- Klatte JL, van der Beek N, Kemperman PMJH. 100 years of Wood's lamp revised — and: Revealing the unseen: a review of Wood's lamp in dermatology. J Clin Aesthet Dermatol. 2024. jcadonline.com.
- Wood lamp skin examination — fluorescence colours and pigment assessment. DermNet NZ. Oakley A. 2014, updated 2024.
- Al Aboud DM, Gossman W. Wood's Light. StatPearls. NCBI Bookshelf NBK537193. 2023.
- Gupta LK, Singhi MK. Wood's lamp. Indian J Dermatol Venereol Leprol. 2004;70(2):131–135.
- Wood's lamp examination: skin analysis under UV light — epidermal vs dermal pigment. Cleveland Clinic. 2024.
- Wood's lamp: how it works and what results mean — false positives and negatives. MSD Manual / clinical review. 2024.
- Asawanonda P, Taylor CR. Wood's light in dermatology. Int J Dermatol. 1999;38(11):801–807.
