A Wood’s lamp is a handheld ultraviolet light used primarily to diagnose skin conditions. It works by shining invisible UV light onto the skin, causing certain infections, pigment changes, and other abnormalities to glow specific colors that aren’t visible under normal lighting. Dermatologists use it as a quick, painless screening tool that takes just a few minutes and requires no needles or skin samples.
How a Wood’s Lamp Works
The lamp emits long-wave ultraviolet light in the 320 to 450 nanometer range, which falls just outside what your eyes can see. When this UV light hits skin, hair, or certain microorganisms, proteins and other molecules absorb the energy and re-emit it as visible light. That re-emitted glow is called fluorescence, and its color depends on what’s causing it. Different bacteria, fungi, and pigment changes each produce a distinct fluorescent signature.
The exam takes place in a fully darkened room. The lamp needs about five minutes to warm up before use, and it’s held roughly two inches from the skin’s surface. The purple glow you see from the lamp itself is normal. What your doctor is looking for is the way your skin responds to that light: bright white patches, coral-red spots, green-yellow hairs, or other telltale colors.
Diagnosing Fungal Infections
One of the most common uses for a Wood’s lamp is identifying fungal infections of the skin and scalp. Not all fungi fluoresce, but several important species do. Infections caused by Microsporum canis and Microsporum ferrugineum (common causes of ringworm on the scalp) glow yellow-green due to byproducts of tryptophan metabolism. Microsporum audouinii, another ringworm species, produces a blue-green fluorescence from a compound called pteridine. Trichophyton schoenleinii, which causes a crusting scalp infection called favus, gives off a dull blue glow.
Pityriasis versicolor, a common yeast-related skin condition that causes light or dark patches on the trunk, fluoresces yellow-orange. This makes it easy to spot even when the patches are subtle under room light.
It’s worth noting that many common fungal species don’t fluoresce at all. A negative Wood’s lamp result doesn’t rule out a fungal infection, which is why doctors sometimes follow up with a skin scraping or culture.
Identifying Bacterial Infections
Certain bacterial infections produce pigments that light up vividly under a Wood’s lamp. The most well-known example is erythrasma, a skin infection caused by Corynebacterium minutissimum that typically appears in skin folds like the groin or armpits. Under a Wood’s lamp, erythrasma glows a distinctive coral-red color because the bacteria produce a compound called coproporphyrin III. This coral-red fluorescence is so characteristic that it can confirm the diagnosis on the spot.
Infections caused by Pseudomonas aeruginosa, a bacterium that can colonize wounds and burns, produce a green fluorescence from an iron-related pigment. Trichobacteriosis, a condition where bacteria coat the hair shafts in the armpits, shows white-yellow fluorescence. These colors help distinguish bacterial infections from fungal ones or simple skin irritation.
Evaluating Pigment Disorders
A Wood’s lamp is one of the first tools a dermatologist reaches for when evaluating white or light patches on the skin, because it can answer a critical question: is the pigment reduced, or completely gone?
In vitiligo, the skin’s pigment-producing cells are destroyed entirely, leaving patches with zero melanin. Under a Wood’s lamp, these depigmented areas fluoresce bright blue-white, standing out sharply even against very pale skin. This enhancement effect is actually required to confirm a vitiligo diagnosis. If white-looking patches don’t brighten or enhance under the lamp, they’re likely hypopigmented (reduced pigment) rather than depigmented (absent pigment), which points toward a different condition entirely.
This distinction matters because the list of conditions that cause lighter patches is long. Nevus depigmentosus, for example, appears as a dull off-white under the lamp rather than the bright blue-white of vitiligo. Tuberous sclerosis produces white ash-leaf shaped spots that also glow white. Meanwhile, melasma (dark patches common during pregnancy or with sun exposure) appears brown or black when the excess pigment is in the upper skin layers, or grey-blue when it sits deeper. This color difference helps guide treatment decisions.
Detecting Porphyrias and Other Conditions
Porphyrias are a group of metabolic conditions where the body accumulates certain light-sensitive compounds. Under a Wood’s lamp, urine, blood, or even teeth from someone with congenital erythropoietic porphyria or porphyria cutanea tarda glow pink to red. This can be a useful early clue when symptoms like blistering skin or photosensitivity raise suspicion.
The lamp also picks up some unexpected findings. Head lice eggs (nits) appear as white or gray spots on the scalp. Milia, the tiny white bumps common on the face, glow bright yellow because of keratin trapped in the skin. Certain skin cancers can also be evaluated: newer Wood’s lamp models tuned to 395 nanometers are used alongside special topical creams to help visualize basal cell carcinomas and precancerous spots called actinic keratoses, with affected areas glowing coral-red.
Veterinary Use for Ringworm
Veterinarians frequently use Wood’s lamps to screen cats and dogs for ringworm, particularly infections caused by Microsporum canis, the most common species in pets. Infected hairs glow green-yellow under the lamp, making it a fast way to check animals in shelters or multi-pet households.
A study of shelter cats found the Wood’s lamp had a sensitivity of 71% and a specificity of 92%. In practical terms, this means it correctly identified about 7 out of 10 infected cats, and when it showed a positive glow, it was right about 9 out of 10 times. The takeaway: a positive result is fairly reliable, but a negative result doesn’t mean the animal is clear. Vets typically confirm with a fungal culture when the lamp is negative but suspicion remains. Fluorescent hairs spotted under the lamp are also useful for targeting exactly which hairs to sample for lab testing.
What Can Cause Misleading Results
Several everyday products fluoresce under UV light and can create false impressions during an exam. Sunscreen glows blue-white, which could be mistaken for a pigment disorder. Salicylic acid, common in acne treatments and chemical peels, fluoresces green. Self-tanning products containing dihydroxyacetone produce a salmon-colored glow. Even the antibiotic tetracycline causes yellow fluorescence on the skin or teeth.
Lint, fabric fibers, and some soaps also glow brightly. This is why clinicians generally ask patients to come to the exam with clean, product-free skin. Residue from creams, lotions, or medications applied to the area can mask a true positive or create a false one.
Quick Reference: Fluorescence Colors
- Bright blue-white: vitiligo or other depigmented skin
- Dull off-white: nevus depigmentosus, ash-leaf spots
- Brown or black: epidermal melasma or hyperpigmentation
- Coral-red: erythrasma (bacterial skin infection)
- Green: Pseudomonas infection
- Blue-green: certain Microsporum fungal infections
- Yellow-green: Microsporum canis ringworm
- Yellow-orange: pityriasis versicolor (yeast)
- Orange-red: progressive macular hypomelanosis (in hair follicles)
- Pink to red: porphyria
- White or gray on scalp: head lice nits