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:

• Bacterial infections
• Fungal infections
• Porphyria (an inherited disorder that causes rashes, blistering, and scarring of the skin)
• Skin coloring changes, such as vitiligo and some skin cancers

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:

• Washing your skin before the test (may cause a false-negative result)
• A room that is not dark enough
• Other materials that glow under the light, such as some deodorants, make-up, soaps, and sometimes lint

Do not look directly into the ultraviolet light, as the light may harm the eye.

Black light test; Ultraviolet light test

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.

Updated by: Elika Hoss, MD, Assistant Professor of Dermatology, Mayo Clinic, Scottsdale, AZ. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.