What Is a Wood’s Lamp Exam?

A Wood's lamp is a device that emits purple or violet UV rays. When placed over your skin, it can help your healthcare provider diagnose conditions that affect the skin and hair, including fungal and bacterial infections, lice, and pigment irregularities.

Normal, healthy skin will not glow under the lamp. Some kinds of bacteria, fungi, and other skin conditions will.

This article explains how a Wood's lamp works to detect skin and hair conditions. It also covers the conditions it can help diagnose and what you can expect during the exam.

Also Known As

The Wood's lamp was named for the physicist who invented it. The exam is also sometimes called:

  • Black light test
  • Ultraviolet light test

How a Wood's Lamp Works

A healthcare provider turns on a Wood's lamp and holds it over areas of your skin or hair to look for signs of an infection or other condition. A magnifying glass over the light provides a close-up look.

Cropped shot of dermatologist using the Wood Lamp for diagnosis of skin condition

Inside Creative House / Getty Images

Healthy skin just looks blue under a Wood's lamp. But bacteria, fungi, and abnormal areas of skin have what's known as fluorescence. This means they absorb the wavelength of the light given off by the lamp and essentially "convert" it to a different wavelength. This makes the affected areas glow.

The color of the glow varies according to the type of skin condition.

Conditions Diagnosed by a Wood’s Lamp

Here are some of the conditions that can be diagnosed using a Wood's lamp and how each one appears under the light:

  • Bacterial infections: Bacterial infections look bright green under a Wood's lamp, especially pseudomonal infections. These infections tend to affect people who are hospitalized or have burns. This type of infection can lead to a dangerous complication called sepsis
  • Erythrasma: A skin infection caused by a bacteria called Corynebacterium minutissimum. It shows up as a coral-pink color under a Wood's lamp.
  • Head lice: Head lice nits show up as white during a Wood's lamp exam, while empty nit cases are gray.  
  • Pigment irregularities: A Wood's lamp can detect abnormal amounts of pigmentation. A lack of pigmentation appears bright blue-white under a Wood's lamp. Excessive pigmentation looks darker, with more defined borders.
  • Pityrosporum folliculitis (fungal acne): This is a yeast infection that affects your hair follicles. It can be found on your chest and the upper part of your back. It is difficult to detect because it resembles acne. It looks yellowish-green under a Wood's lamp.
  • Porphyria: A rare group of diseases that can affect your skin and nervous system. People living with porphyria are extra-sensitive to the sun and have burns on their skin. Porphyria is usually a reddish-pink color under the Wood's lamp. 
  • Ringworm: A fungal infection that usually causes itchy round red patches. Despite the name, worms do not cause this infection. Ringworm looks blue-green under the Wood's lamp.
  • Vitiligo: A skin disorder that removes the skin’s color in patches by killing melanin-producing cells. Vitiligo will have sharp borders under a Wood's lamp and will appear bright blue-white or yellow-green.

What to Expect

A Wood's lamp exam is non-invasive, painless, quick, and safe. Here's what to know about how this exam is done.

Who Does the Test

Any type of medical professional can perform a Wood's lamp exam, but these exams are most often done by a dermatologist—a physician who specializes in skin disorders—in their office.

Estheticians may also use these lamps to check for signs of aging like uneven skin tone, wrinkles, and age spots before beginning a cosmetic treatment. While these professionals specialize in providing skincare treatments, they are not medical professionals and cannot formally diagnose you with any condition.

How to Prepare

Your skin should be clean and dry before the exam. Do not apply any perfumes, makeup, lotions, or other skincare products.

These may show up under the light and affect the results, either indicating there may be a problem when there is not or making something that truly is a concern less noticeable.

During the Test

A Wood's lamp exam must be performed in a completely dark room.

First, your healthcare provider will turn on the lamp and let it warm up for about a minute. Then they will hold the lamp about 4 to 5 inches away from your skin and hair and examine each area for a couple of seconds.

You will be asked to cover your eyes or wear special goggles during a Wood's lamp exam to avoid damage to your corneas, the dome-like structures that help your eyes focus.

Interpreting Results

Although any medical professional can do a Wood's lamp exam, it is highly recommended that a dermatologist interprets the results.

After reviewing your symptoms and conducting a Wood's lamp exam, your doctor may have enough information to diagnose your skin condition and move ahead with treatment.

This chart summarizes the possibilities:

 Condition  Color Under Wood's Lamp
 Bacterial infections  Bright green
 Erythrasma  Coral/pink
 Head lice  White or gray
 Pigment irregularities  Bright blue/white
 Pityrosporum folliculitis  Yellow/green
 Porphyria  Reddish pink
 Ringworm  Blue/green
 Vitiligo  Bright blue/white or yellow/green

It's also possible they may need to perform other types of tests to get an accurate diagnosis.

Summary

A Wood's lamp emits long UV rays that can help identify various skin conditions. When the light shines on the skin and hair, fungal and bacterial infections, pigmentation abnormalities, lice, and other conditions will illuminate. The colors that they "light up" can guide a diagnosis.

A Wood's lamp exam is brief, painless, and safe. If your doctor diagnoses you with a skin condition, the next step will to receive the appropriate treatment.

Frequently Asked Questions

  • Are there any risks associated with using a Wood’s lamp?

    There are no significant risks associated with a Wood’s lamp examination. The type of light used does not irritate or burn the skin. You will be asked to cover your eyes during the exam to avoid any damage to the cornea.

  • How should normal skin look under a Wood’s lamp?

    Healthy skin doesn’t glow under a Wood’s lamp and appears blue.

  • What’s the difference between a Wood’s lamp and a black light?

    The light these devices emit is the same, though they are constructed differently. A Wood's lamp exam is sometimes called a black light test.

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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. DermNet NZ. Wood lamp skin examination.

  2. Sharma S, Sharma A. Robert Williams Wood: pioneer of invisible lightPhotodermatol Photoimmunol Photomed. 2016;32(2):60-65. doi:10.1111/phpp.12235

  3. University of Michigan Health. Pseudomonas infection.

  4. Bonilla C, Ness AR, Wills AK, Lawlor DA, Lewis SJ, Davey Smith G. Skin pigmentation, sun exposure and vitamin D levels in children of the Avon Longitudinal Study of Parents and Children. BMC Public Health. 2014;14(1):597.

  5. Mount Sinai. Wood's lamp examination.

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