Wood's lamp examination

Definition

A Wood's lamp examination is a test that uses ultraviolet light to closely look at the skin.

Alternative Names

Black light test; Ultraviolet light test

How the test is performed

The test is done while you are seated in a dark room, usually in a dermatologist's office. The health care provider turns on the Wood's lamp, holds it 4 to 5 inches from the area of skin being studied, and looks for any skin color changes.

You should not look directly into the light.

How to prepare for the test

No special preparation is needed. If you are treating the area in question with any topical medications, you may wish to skip an application before visiting the doctor.

How the test will feel

You will feel nothing during this test.

Why the test is performed

Your health care provider may perform this test to detect several conditions affecting the skin, including

• Bacterial infections
• Fungal infections
• Porphyria
• Skin coloring changes

Normal Values

Normally your skin will not shine, or fluoresce, under the ultraviolet light.

What abnormal results mean

A Wood's lamp exam may help your doctor confirm a fungal infection or bacterial infection. Your doctor may also be able to learn what is causing any light- or dark-colored spots on your skin.

What the risks are

There are no risks. Avoid looking directly into the ultraviolet light.

Special considerations

Do not wash before the test, because that may cause a false-negative result. A room that is not dark enough may also alter results. Other materials may also glow. For example, some deodorants, make-ups, soaps, and even lint may be visible with the Wood's lamp.

Not all types of bacteria and fungi can be detected with the light.

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References

Morelli JG. Evaluation of the patient. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 644.

Harrison S, Piliang M, Bergfeld W. Hair disorders. In: Carey WD, ed. Cleveland Clinic: Current Clinical Medicine 2010. 2nd ed. Philadelphia, Pa: Saunders Elsevier; 2010.