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Wood's lamp examination

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 sit in a dark room for this test. The test is usually done in a skin doctor's (dermatologist) office. The doctor will turn on the Wood's lamp and hold it 4 to 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. Follow your doctor's instructions about not putting creams or medicines on the area of the skin before the test.

How the Test will Feel

You will have no discomfort during this test.

Why the Test is Performed

This test is done to look for skin problems including:

  • Bacterial infections
  • Fungal infections
  • Porphyria
  • Skin coloring changes, such as vitiligo

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

Normal Results

Normally the 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.

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


There are no risks with this test. DO NOT look directly into the ultraviolet light.

Alternative Names

Black light test; Ultraviolet light test


Gebhard RE. Wood's light examination. In: Pfenninger JL, Fowler GC, eds. Pfenninger & Fowler's Procedures for Primary Care. 3rd ed. Philadelphia, PA: Elsevier Mosby; 2010:chap 43.

Ruocco E, Baroni A, Donnarumma G, Ruocco V. Diagnostic procedures in dermatology. Clin Dermatol. 2011;29:548-556. PMID 21855731 www.ncbi.nlm.nih.gov/pubmed/21855731.

Update Date 12/2/2014

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