Wood's lamp examination

Alternative Names

Black light test; Ultraviolet light test

Definition

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

How the Test is Performed

You will sit in a dark room for this test. The test usually takes place in a dermatologist's office. The health care provider will turn on the Wood's lamp and hold it 4 - 5 inches from the skin to look for color changes.

How to Prepare for the Test

You do not need to take any special steps before this test. Ask your doctor if you avoid putting creams or medicines on the area of the skin being studies before the test.

How the Test Will Feel

You will feel nothing during this test.

Why the Test is Performed

Your health care provider may do this to look for skin problems including:

Bacterial infections
Fungal infections
Porphyria
Skin coloring changes such as vitiligo

Normal Results

Normally your skin will not shine 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.

Risks

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

Considerations

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-ups, soaps, and sometimes lint

Not all types of bacteria and fungi show up under the light.

References

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.

Morelli JG. Evaluation of the patient. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 637.