A Wood’s lamp is a handheld device that emits ultraviolet (UV) light to reveal skin conditions invisible to the naked eye. When shone on the skin in a dark room, it causes certain bacteria, fungi, and pigment changes to glow in distinctive colors, helping clinicians identify infections and other skin problems quickly and painlessly.
How a Wood’s Lamp Works
The lamp emits UV-A light in the range of 320 to 400 nanometers, with a peak wavelength of 365 nanometers. That places it just outside the visible spectrum, in the “blacklight” range most people recognize from nightclubs or glow-in-the-dark posters. The physics behind it are straightforward: when UV light hits certain substances on or in the skin, those substances absorb the UV energy and release it back as visible light. This process is called fluorescence.
Different substances fluoresce in different colors. A bacterial infection might glow coral-red, while a fungal infection might appear bright green. Healthy, unaffected skin typically looks a dull blue or purple under the lamp. By reading these color differences, a clinician can narrow down what’s causing a rash, patch, or wound infection without waiting days for a lab culture to come back.
The lamp itself is named after Robert Williams Wood, an American physicist who invented “Wood’s glass,” a special filter that blocks visible light while allowing ultraviolet light to pass through. His work in UV photography eventually found its way into medicine, where the device became a standard bedside diagnostic tool.
What It Can Detect
Wood’s lamps are used across a surprisingly wide range of skin conditions. Their most common applications fall into three categories: infections, pigmentation disorders, and wound assessment.
Bacterial and Fungal Infections
Certain bacteria produce natural chemicals that fluoresce under UV light. Pseudomonas, a bacterium that can infect burns and chronic wounds, secretes a substance called fluorescein that glows green under a Wood’s lamp. This is useful in wound care, where spotting Pseudomonas early can change the course of treatment. Erythrasma, a common bacterial skin infection that causes reddish-brown patches in skin folds, fluoresces a distinctive coral-red color. That coral glow is often enough to distinguish it from fungal infections that look nearly identical under normal light.
Some fungal infections also respond to the lamp. Certain species of the fungus that causes ringworm will glow a bright blue-green. However, not all species fluoresce, which means a negative result under the lamp doesn’t rule out a fungal infection entirely. Additional testing, like a skin scraping or culture, is sometimes still needed.
Pigmentation Changes
One of the lamp’s most valuable uses is making subtle pigmentation changes more visible. In conditions like vitiligo, where patches of skin lose their pigment, the contrast between affected and unaffected skin becomes dramatically more obvious under UV light. This helps clinicians map the full extent of pigment loss, including areas so faint they’d be easy to miss in regular lighting.
The lamp can also help distinguish where pigment changes are happening in the skin. Pigment sitting in the outermost layer of skin (the epidermis) tends to become more pronounced under the lamp, while pigment deeper in the skin (the dermis) shows less contrast. This distinction matters for conditions like melasma, where the depth of pigment affects which treatments are likely to work.
Wound and Tissue Assessment
Beyond infections and pigmentation, clinicians sometimes use the lamp to check for the presence of certain body fluids, to assess the borders of skin lesions, or to evaluate how well a wound is healing. The lamp can also highlight areas of skin that have been exposed to certain chemicals or irritants that wouldn’t otherwise be visible.
What the Exam Feels Like
A Wood’s lamp exam is completely painless and takes only a few minutes. You sit or lie down in a completely darkened room while the clinician holds the lamp a few inches from your skin. The room needs to be genuinely dark, not just dim. Even a small amount of ambient light can wash out the fluorescence and make results unreliable.
The lamp takes about a minute to warm up, and your eyes need roughly the same amount of time to adjust to the darkness. During the exam, the clinician slowly moves the light across the area of concern, looking for any color changes. You won’t feel heat or discomfort from the light. The UV-A wavelength used is the same type found in ordinary sunlight, and at the low intensity and short duration of an exam, it poses no meaningful risk to your skin.
What Can Affect the Results
Several things can cause misleading results during a Wood’s lamp exam. The most common issue is the room not being dark enough, which can mask subtle fluorescence. But certain products on the skin can also create false signals. Some soaps, cosmetics, topical medications, and even lint from clothing can fluoresce under UV light, mimicking the glow of an infection or skin condition that isn’t actually there. For this reason, you may be asked to wash the area being examined beforehand and avoid applying lotions or creams.
On the other side, some genuine infections won’t fluoresce at all. As noted with fungal infections, only certain species produce the chemicals that glow under UV. A Wood’s lamp is a useful screening tool, but clinicians treat it as one piece of the puzzle rather than a definitive answer on its own.
Use in Veterinary Medicine
If you’ve ever taken a cat or dog to the vet for a suspected ringworm infection, there’s a good chance a Wood’s lamp was involved. The fungal species most commonly responsible for ringworm in cats, Microsporum canis, fluoresces under UV light. However, the lamp’s sensitivity for detecting this fungus is only about 37.5%, meaning it misses roughly six out of ten infected animals. Its specificity is much higher at 96.1%, so when it does glow, the result is quite reliable. Because of that gap, a negative result under the lamp doesn’t mean a pet is infection-free, and vets typically follow up with a fungal culture for confirmation.