Black light test; Ultraviolet light test A Wood lamp examination is a test that uses ultraviolet (UV) light to look at the skin closely. A Wood's lamp is a light that uses long wave ultraviolet light. When an area of scalp that is infected with tinea (a type of ringworm fungus) is viewed under a Wood's light, the fungus may glow. This test may be done to detect the presence of a fungal scalp or skin infection. A Wood's lamp emits ultraviolet light and can be a diagnostic aid in determining if someone has a fungal or bacterial infection on the skin or scalp. If there is an infection on the area where the
Wood's lamp is illuminating, the area will fluoresce. Normally the skin does not fluoresce, or shine, under ultraviolet light. You sit in a dark room for this test. The test is usually done in a skin doctor's (dermatologist's) office. The doctor will turn on the Wood lamp and hold it 4 to 5 inches (10 to 12.5 centimeters) from the skin to look for color
changes. You do not need to take any special steps before this test. Follow your doctor's instructions about not putting creams or medicines on the area of the skin before the test. You will have no discomfort during this test. This test is done to look for skin problems including: Not all types of bacteria and fungi show up under the light. Normally the skin will not shine under the ultraviolet light. A Wood lamp exam may help your doctor
confirm a fungal or bacterial infection or diagnose vitiligo. Your doctor may also be able to learn what is causing any light- or dark-colored spots on your skin. The following things can change the results of the test:How the Test Is Performed
How to Prepare for the Test
How the Test will Feel
Why the Test Is Performed
Normal Results
What Abnormal Results Mean
Risks
DO NOT look directly into the ultraviolet light, as the light may harm the eye.
References
Dinulos JGH. Light-related diseases and disorders of pigmentation. In: Dinulos JGH, ed. Habif's Clinical Dermatology: A Color Guide to Diagnosis and Therapy. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 19.
Spates ST. Diagnostic techniques. In: High WA, Prok LD, eds. Dermatology Secrets. 6th ed. Philadelphia, PA: Elsevier; 2021:chap 3.
Version Info
Last reviewed on: 11/4/2020
Reviewed by: Elika Hoss, MD, Senior Associate Consultant, Mayo Clinic, Scottsdale, AZ. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
The Wood's light or Wood's lamp was invented by a physicist named Robert Wood. It's a light that emits long-wave ultraviolet light and can detect fluorescence in skin and hair. A Wood's lamp can be used to detect bacterial or fungal skin infections. It can also be used to detect skin pigment irregularities, like white patches
on your skin called vitiligo, or corneal abrasions, which are scratches on your eye. It is sometimes used to select infected skin or hair for lab tests. A Wood's lamp exam is painless and safe. It involves the following steps: The Wood's lamp exam shows areas of skin that are pigmented,
depigmented, or fluorescent. Normal, healthy skin appears blue under a Wood's lamp and does not glow. It may show white spots where the skin is thick, yellow spots where it's oily, or purple where it's dehydrated. A positive result means there are obvious differences in your skin's appearance. A Wood's lamp can diagnose a variety of conditions, including: Tinea capitis.
Tinea capitis is a fungal infection that causes areas of baldness and scaling. The fungal species that cause tinea capitis look blue-green or dull blue under the Wood's lamp. Fungal infections caused by other species don't show up. Vitiligo. Vitiligo is a
skin disease where the skin loses its color in patches. Melanin produces skin color. Vitiligo occurs in areas where cells that produce melanin die or stop functioning. A Wood's lamp can identify affected areas in light-skinned people. These areas will have sharper borders under black light and will look bright blue-white or yellow-green. How Is a Wood's Lamp Exam Performed?
What Does a Wood's Lamp Exam Show?
What Can a Wood's Lamp Exam Diagnose?
Ringworm. Like tinea capitis, ringworm, or tinea corporis, is a skin infection caused by a fungus. It's a red, itchy, circular rash that gets its name from its appearance. There are no worms involved. Porphyria.
Porphyria is a group of eight inherited disorders that affect the skin and nervous system. People with porphyria that affects the skin are often sensitive to sunlight and may have abrasions and blisters on their skin. Under a Wood's lamp, porphyria shows up as red-pink. Pigment disorders. A Wood's lamp can detect other changes in
your skin's pigmentation besides vitiligo. Hypopigmentation, or loss of pigmentation, can be difficult to see in fair-skinned people. Under a Wood's lamp, areas of hypopigmentation have sharper borders and show up as bright blue-white. Hyperpigmentation, which has more melanin than normal, also shows up with sharper borders under a Wood's lamp because areas with more melanin absorb
more light. Pityrosporum folliculitis. This is an infection of the hair follicles caused by yeast. It's normally on the upper back and chest. Because it can look like acne, it can be difficult to diagnose. Under a Wood's lamp, it will look yellow-green. Bacterial infections. Infections from bacteria like Pseudomonas look bright green under a Wood's lamp. Pseudomonas is especially dangerous in burn wounds since it's hard to treat. It can lead to a fatal complication of infection called sepsis. Head lice. Nits from
head lice can be difficult to tell from dried hairspray or hair casts. Live nits glow white under a Wood's lamp, and empty nit cases look gray. Doctors are not the only professionals who use Wood's lamps. Here are some of their other health-related uses:Other Uses for a Wood's Lamp
Safety Concerns
A Wood's lamp is safe to use. It doesn't damage the skin or cause sunburn. The examination is so brief that it's unlikely to cause irritation even in the most light-sensitive people. But if you're having a Wood's lamp exam done, you should close your eyes to avoid any risk to your cornea.