The Wood's Lamp, Explained: Why a 1903 Invention Still Settles Modern Vitiligo Questions

Walk into almost any dermatology exam room with a pale patch on your skin, and one of the first things the clinician will do is turn off the lights and switch on a small violet-glowing device. The Wood's lamp, developed by physicist Robert Wood in 1903, remains one of the most decisive tools in evaluating light spots on the skin. Understanding what it does helps patients understand why the same-looking patch can lead to very different diagnoses and very different treatment plans.

How the lamp actually works. A Wood's lamp emits long-wave ultraviolet A light, centered around 365 nanometers, filtered through nickel oxide glass to remove most visible light. When this UVA light hits skin, two things happen. First, melanin in the epidermis absorbs it. Second, certain molecules in the dermis, particularly collagen cross-links, fluoresce, meaning they absorb the UV energy and re-emit it as visible bluish-white light. In normally pigmented skin, epidermal melanin acts as a curtain, absorbing much of the light before it reaches the fluorescent dermis below.

Why vitiligo glows. In vitiligo, melanocytes are absent from affected patches, so there is no melanin curtain at all. UVA light passes straight through the epidermis, strikes dermal collagen, and bounces back as a sharp, bright, chalk-white or blue-white glow with crisp borders. This is the signature of true depigmentation, a complete loss of pigment cells rather than a reduction in their output.

Why other conditions do not. Here is where the lamp earns its keep. Many conditions cause hypopigmentation, meaning melanocytes are still present but producing less melanin. Pityriasis alba, common in children with eczema-prone skin, shows only a subtle, off-white accentuation under the lamp because residual melanin still absorbs some light. Post-inflammatory hypopigmentation after burns, rashes, or procedures behaves similarly. Tinea versicolor, a yeast overgrowth, often gives itself away with a yellow-green or copper-orange fluorescence produced by the organism itself. Nevus depigmentosus, a stable birthmark-like patch, typically shows moderate enhancement but not the stark chalk-white of vitiligo, and its borders often look irregular or feathered rather than sharp.

The stakes of getting this right. The distinction is not academic. Vitiligo is an autoimmune condition in which the immune system, driven substantially by interferon-gamma signaling and CD8-positive T cells, targets melanocytes. Its treatments, including topical corticosteroids, calcineurin inhibitors, JAK inhibitors, and narrowband UVB phototherapy, are aimed at calming that immune attack and coaxing melanocyte reserves out of hair follicles to repopulate the skin. Pityriasis alba, by contrast, usually needs only moisturization, mild anti-inflammatory therapy, and time. Tinea versicolor needs an antifungal. Treating a fungal condition with immune-modulating creams, or subjecting a self-resolving childhood condition to months of phototherapy, helps no one.

What the lamp reveals about disease activity. Beyond diagnosis, the Wood's lamp helps clinicians assess whether vitiligo is stable or spreading. Under UVA light, some patients show faint depigmented margins extending beyond what is visible in room light, sometimes described as a trichrome pattern with zones of full, partial, and normal pigmentation. Poorly defined, hazy borders under the lamp can suggest active disease, which influences whether a clinician prioritizes stabilizing treatment before attempting repigmentation. In patients with very fair skin, where patches are nearly invisible in ordinary light, the lamp may be the only way to map the true extent of involvement.

What the lamp cannot do. It is worth being clear about limits. A Wood's lamp examination is a screening and characterization tool, not a definitive test. It cannot confirm the autoimmune mechanism of vitiligo, cannot distinguish every mimic, and can be confounded by room light leakage, topical products, and even lint, which fluoresces brightly and startles many first-time patients. Conditions such as hypopigmented mycosis fungoides, a rare lymphoma of the skin, or leprosy in endemic regions may still require biopsy to exclude. Ambiguous cases, patches with unusual texture, scaling, numbness, or rapid change, warrant tissue diagnosis regardless of what the lamp shows.

The takeaway for patients. If you are being evaluated for a light patch, expect the lights to go off. A bright, sharply bordered white glow points toward vitiligo and its immune-directed treatment pathway. A muted enhancement points toward hypopigmentation with intact melanocytes, which often carries a gentler prognosis and simpler management. Either way, the answer shapes everything that follows. More than a century after its invention, this simple filtered bulb still does what expensive technology often cannot: it makes an invisible biological difference, the presence or absence of melanocytes, visible in seconds, at no risk to the patient. Few tools in medicine offer that ratio of insight to cost.

Related reading: Not Every White Patch Is Vitiligo: How Clinicians Tell Four Common Look-Alikes Apart.

Further reading: The immunology of vitiligo (Nat Rev Immunol 2026); Vitiligo (Ugeskr Laeger 2025); Comorbidities in Patients with Vitiligo: A Systematic Review and Meta-Analysis (J Invest Dermatol 2023).