Wood’s lamp

Alternative names
Black light test; Ultraviolet light test

Wood’s lamp is a test that is performed in a dark room where ultraviolet light is shined on the area of interest.

How the test is performed
You will be seated and the lights will be turned off. After your eyes adjust to the darkness, your physician will turn on the Wood’s lamp and hold it 4 to 5 inches from the area being studied. Your physician will then look at your skin for any color changes. (Take care not to 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. Likewise, there should be no adverse side effects.

Why the test is performed
Your health care provider may perform this test to detect several conditions, including:

  • Bacterial infections  
  • Fungal infections  
  • Porphyria  
  • Pigmentary alterations

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

What abnormal results mean

Using the light, your doctor may be able to confirm a diagnosis of fungal infection or bacterial infection. Your doctor may also be able to gain insight into the cause of light or dark colored spots on your skin.

What the risks are
There are no risks, but avoid looking directly into the ultraviolet light, as you avoid looking into the sun.

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 infections can be detected with the light. Some species of fungi and bacteria do not contain fluorescent chemicals.

Johns Hopkins patient information

Last revised: December 8, 2012
by Brenda A. Kuper, M.D.

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