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What is a wood’s lamp for dermatology?

Wood’s lamps, also known as blacklight lamps, are specialized ultraviolet lamps used in dermatology to help diagnose certain skin conditions. In this comprehensive guide, we’ll explore what exactly a wood’s lamp is, how it works, what skin conditions it can help identify, and how dermatologists use it in clinical practice.

What is a Wood’s Lamp?

A wood’s lamp is a lamp that emits ultraviolet light in the long wave (UV-A) spectrum. Unlike shorter wavelength UV light, such as UV-B and UV-C, long wave UV-A is not as damaging to human skin and eyes. Wood’s lamps were invented in 1903 by American physicist Robert Williams Wood, hence the name “wood’s lamp.”

Today, wood’s lamps used in dermatology typically emit ultraviolet light at a peak wavelength of 365 nm. At this wavelength, certain substances in or on the skin will absorb the UV light and fluoresce, or glow. This fluorescence is invisible to the naked eye under normal lighting conditions. But under the wood’s lamp, these fluorescent areas become visible and help identify certain skin conditions.

How Does a Wood’s Lamp Work?

A wood’s lamp works by emitting long wave UV-A light, which is invisible to the human eye. Certain molecules, called chromophores, will absorb this UV light and then emit light back at a different wavelength in the visible spectrum, causing fluorescence. Common fluorescent chromophores in the skin include:

  • Porphyrins – byproducts of bacteria
  • Endogenous fluorophores – molecules naturally present in the skin, like collagen and elastin
  • Exogenous fluorophores – molecules applied topically to the skin in creams, makeup, etc.

When a dermatologist shines the UV light from a wood’s lamp onto a patient’s skin, any areas containing increased levels of these fluorescent chromophores will glow, helping identify skin conditions characterized by their presence. The color of the fluorescence depends on the specific molecules causing it.

What Skin Conditions Can a Wood’s Lamp Detect?

A wood’s lamp can help diagnose a range of dermatological conditions by causing fluorescent areas indicative of the disease to become visible. Some examples include:

  • Tinea Capitis – A fungal infection (ringworm) of the scalp that can cause fluorescent scales.
  • Vitiligo – Autoimmune condition causing depigmentation, visible as areas lacking fluorescence.
  • Porphyrias – Disorders causing accumulation of fluorescent porphyrins in the skin.
  • Pityriasis Versicolor – Fungal infection that fluoresces yellow green under wood’s lamp.
  • Erythrasma – Bacterial skin infection that fluoresces bright coral-red.
  • Acne and Seborrheic Dermatitis – Skin fluoresces yellow due to presence of Propionibacterium acnes bacteria.
  • Skin Cancers – Some may take up topical photosensitizers and become fluorescent.

In addition to diseases, the wood’s lamp can also help identify exogenous fluorescent substances on the skin, like skin creams, soaps, and makeup. It can also detect traces of drugs or chemicals not visible under normal lighting.

How Do Dermatologists Use a Wood’s Lamp?

Dermatologists use a wood’s lamp as an auxiliary diagnostic tool during patient exams. It serves as a quick, non-invasive way to get additional information about skin lesions or rashes that may not be obvious under normal lighting. Here are some examples of how it is used in clinical practice:

  • Screen for fungal infections – Shine lamp over body to look for characteristic fluorescent scaling or lesions.
  • Delineate margins – Define borders of skin lesions that fluoresce differently from surrounding skin.
  • Monitor spreading – See if a rash or lesion is spreading by looking for increasing fluorescence.
  • Aid biopsy – Choose best site for biopsy by targeting fluorescent areas.
  • Assess treatment – Check if antifungal creams are covering entire affected area based on fluorescence.
  • Detect contamination – See if skin is contaminated by chemicals, oils, makeups, topical drugs, etc.

However, while it can provide useful information, a wood’s lamp alone is not diagnostic. Dermatologists will correlate wood’s lamp findings with patient history, clinical presentation, and sometimes lab tests or biopsies for definitive diagnoses. It remains an adjunctive tool to aid in examination and workup.

Interpreting Findings from a Wood’s Lamp Exam

Learning to properly interpret wood’s lamp findings takes clinical experience. However, some basic patterns can help dermatologists narrow down a potential diagnosis:

Fluorescence Color What it Indicates
Blue-white Endogenous skin fluorescence, often indicating normal skin.
Yellow Presence of Propionibacterium acnes bacteria or endogenous skin lipids.
Yellow-green Pityriasis versicolor fungal infection.
Red Erythrasma bacterial infection or porphyrias.
Orange Fungal infection like tinea capitis.
Pink Can indicate porphyrias, Candida fungi, or skin contact with oils.
Purple Presence of Pseudomonas bacteria.
Chalky white Vitiligo or autoimmune depigmentation.

However, the same color fluorescence could have multiple causes. Proper diagnosis integrates the specific distribution and location of fluorescence with clinical context.

Advantages of Using a Wood’s Lamp for Skin Diagnosis

There are several advantages that make a wood’s lamp a useful diagnostic tool in dermatology:

  • Non-invasive – It does not cause any pain or require samples to be taken from the patient.
  • Quick – Large body surface areas can be scanned rapidly.
  • Low cost – Wood’s lamps are relatively inexpensive pieces of equipment.
  • Highlights contrasts – Fluorescence helps delineate the borders of skin lesions.
  • No preparation needed – Can be used directly on a patient without any prior skin preparation.
  • Aids clinical exam – Provides additional diagnostic information not visible under normal lighting.

Limitations of Wood’s Lamps in Dermatology

However, wood’s lamps also have some limitations, including:

  • Low specificity – The same fluorescence pattern can have multiple causes requiring further testing.
  • User-dependent – Results rely heavily on proper usage and interpretation by the dermatologist.
  • Not diagnostic alone – Findings must correlate with clinical context; cannot independently confirm diagnoses.
  • Age-dependent effects – Skin fluorescence changes naturally with aging.
  • Does not detect all conditions – Many disorders do not exhibit fluorescence.

Overall, the wood’s lamp serves best as an adjunct to help dermatologists synthesize clinical and historical information for diagnosis – not as a diagnostic tool by itself. Proper training is required to use it effectively.


In summary, a wood’s lamp is a specialized ultraviolet lamp used in dermatology to visualize fluorescent structures in the skin that are invisible under normal lighting. It works by emitting long wave UV-A light, which causes fluorescent chromophores in the skin to absorb the light and re-emit it as visible fluorescence. This can help diagnose various fungal infections, bacterial colonization, autoimmune conditions like vitiligo, and exogenous substances on the skin.

Dermatologists use wood’s lamps as an auxiliary non-invasive tool during skin exams to aid in diagnosis and monitoring. However, findings must correlate with the clinical context as wood’s lamp testing alone cannot definitively confirm most diagnoses. With proper training and interpretation, it remains a valuable instrument in the dermatologist’s toolbox for synthesizing clinical information and narrowing down differential diagnoses.