Black light test; Ultraviolet light test
A Wood lamp examination is a test that uses ultraviolet (UV) light to look at the skin closely.
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 lamp and hold it 4 to 5 inches (10 to 12.5 centimeters) 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 (an inherited disorder that causes rashes, blistering, and scarring of the skin)
- Skin coloring changes, such as vitiligo
Not all types of bacteria and fungi show up under the light.
Normally the skin will not shine under the ultraviolet light.
What Abnormal Results Mean
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
There are no risks with this test. DO NOT look directly into the ultraviolet light.
Gebhard RE. Wood's light examination. In: Pfenninger JL, Fowler GC, eds. Pfenninger and Fowler's Procedures for Primary Care. 3rd ed. Philadelphia, PA: Elsevier Mosby; 2011:chap 43.
Habif TP. Light-related diseases and disorders of pigmentation. In: Habif TP, ed. Clinical Dermatology: A Color Guide to Diagnosis and Therapy. 6th ed. Philadelphia, PA: Elsevier; 2016:chap 19.