What Is a Wood's Lamp Exam?

Medically Reviewed by Debra Jaliman, MD on July 23, 2023
Written by WebMD Editorial Contributors
4 min read

The Wood's light or Wood's lamp was invented by a physicist named Robert Wood. It's a light that emits long-wave ultraviolet light and can detect fluorescence in skin and hair. A Wood's lamp can be used to detect bacterial or fungal skin infections. It can also be used to detect skin pigment irregularities, like white patches on your skin called vitiligo, or corneal abrasions, which are scratches on your eye. It is sometimes used to select infected skin or hair for lab tests.

A Wood's lamp exam is painless and safe. It involves the following steps:

  • Your skin should be clean but not recently washed and free of any deodorant, makeup, or creams.
  • Your healthcare worker will turn on the Wood's lamp to warm up for about a minute. 
  • Then, they will turn off the room lights and close any shades to darken the room completely. 
  • Your provider will hold the Wood's lamp several inches away from your skin or hair and examine it for a few seconds.

The Wood's lamp exam shows areas of skin that are pigmented, depigmented, or fluorescent. Normal, healthy skin appears blue under a Wood's lamp and does not glow. It may show white spots where the skin is thick, yellow spots where it's oily, or purple where it's dehydrated. A positive result means there are obvious differences in your skin's appearance. 

A Wood's lamp can diagnose a variety of conditions, including:

Tinea capitis. Tinea capitis is a fungal infection that causes areas of baldness and scaling. The fungal species that cause tinea capitis look blue-green or dull blue under the Wood's lamp. Fungal infections caused by other species don't show up. 

Vitiligo. Vitiligo is a skin disease where the skin loses its color in patches. Melanin produces skin color. Vitiligo occurs in areas where cells that produce melanin die or stop functioning. A Wood's lamp can identify affected areas in light-skinned people. These areas will have sharper borders under black light and will look bright blue-white or yellow-green. 

Ringworm. Like tinea capitis, ringworm, or tinea corporis, is a skin infection caused by a fungus. It's a red, itchy, circular rash that gets its name from its appearance. There are no worms involved.

Porphyria. Porphyria is a group of eight inherited disorders that affect the skin and nervous system. People with porphyria that affects the skin are often sensitive to sunlight and may have abrasions and blisters on their skin. Under a Wood's lamp, porphyria shows up as red-pink. 

Pigment disorders. A Wood's lamp can detect other changes in your skin's pigmentation besides vitiligo. Hypopigmentation, or loss of pigmentation, can be difficult to see in fair-skinned people. Under a Wood's lamp, areas of hypopigmentation have sharper borders and show up as bright blue-white. Hyperpigmentation, which has more melanin than normal, also shows up with sharper borders under a Wood's lamp because areas with more melanin absorb more light. 

Pityrosporum folliculitis. This is an infection of the hair follicles caused by yeast. It's normally on the upper back and chest. Because it can look like acne, it can be difficult to diagnose. Under a Wood's lamp, it will look yellow-green. 

Bacterial infections. Infections from bacteria like Pseudomonas look bright green under a Wood's lamp. Pseudomonas is especially dangerous in burn wounds since it's hard to treat. It can lead to a fatal complication of infection called sepsis. 

Head lice. Nits from head lice can be difficult to tell from dried hairspray or hair casts. Live nits glow white under a Wood's lamp, and empty nit cases look gray. 

Doctors are not the only professionals who use Wood's lamps. Here are some of their other health-related uses:

  • Molecular biology labs may use a Wood's lamp to detect compounds with a fluorescent tag.
  • Ophthalmologists can use one to look for scratches and foreign objects in the cornea of the eye.
  • Estheticians use them to check for signs of aging skin and other imperfections.
  • Veterinarians use a Wood's lamp to check pets for bacterial, fungal, or parasitic infections.

A Wood's lamp is safe to use. It doesn't damage the skin or cause sunburn. The examination is so brief that it's unlikely to cause irritation even in the most light-sensitive people. But if you're having a Wood's lamp exam done, you should close your eyes to avoid any risk to your cornea.