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Can Fam Physician. 2012 September; 58(9): 976.
PMCID: PMC3440273

Wood lamp examination

David Ponka, MD CM CCFP(EM) FCFP
Associate Professor in the Department of Family Medicine at the University of Ottawa in Ontario.
Faisal Baddar, MD CM CCFP


None. Cover patients’ eyes to prevent injury to the retina or conjunctiva.


Diagnosis of numerous dermatologic disorders as well as porphyrias.

Equipment necessary

Wood lamp.


This procedure works best in a completely dark environment, so turn off lights and close any window shades in the room. Allow the Wood lamp to warm up (about 1 minute), and give your eyes time to adapt to the dark.1


Shine the light directly above the lesion or sample in question. Carefully inspect the area, paying close attention to zones in fluorescence and their borders. Watch out for lint, topical medications, and soap residue! They all fluoresce, creating false positives, usually red-pink.2 Thankfully, they can generally be removed.


While no epidemiologic evidence is currently available, physiologic evidence is well established. A Wood lamp is a mercury arc covered by a Wood filter, which allows a narrow range of light to escape: 320 to 400 nm.2 The 2 main substances that fluoresce are dermal collagen and porphyrins.

Diagnostic confirmation

  • Pigment disorders: biopsy.
  • Bacterial infections: culture.
  • Fungal infections: potassium hydroxide microscopy, culture.2
  • Porphyrias: urine testing for porphobilinogen and 5-aminolevulinic acid, genetic testing.3

Pigment disorders1,3

Hypopigmentation or depigmentation

Lesions demonstrate increased sharpness of borders under Wood lamp examination and fluoresce bright blue-white owing to the increased amount of dermal collagen illuminated. This results from decreased or absent intervening melanin.

Vitiligo: patches, varying in location and extent.

Tuberous sclerosis: patches; characteristic shapes are lanceolate or ash-leaf. Other far-less-specific shapes exist.

Hypomelanosis of Ito: whirled or streaked patterns.


Lesions demonstrate enhanced border contrast under Wood lamp examination owing to increased absorption of light by increased amounts of melanin.


If of epidermal origin, lesions are well demarcated. Dermal melasma demonstrates less colour contrast.



Pseudomonas species: green in folliculitis and infected burn wounds.

Corynebacterium minutissimum: coral red in erythrasma.

Propionibacterium acnes: orange-red in comedones.


Tinea versicolor: Malassezia furfur yellowish-white or copper-orange.

Pityrosporum folliculitis: Bluish-white in a follicular pattern.

Tinea capitis: Fluoresces in less than 5% of cases in the United States; blue-green (most Microsporum species), occasionally dull yellow (Microsporum gypseum) and dull blue (Trichophyton schoenleinii).


These disorders all cause red-pink fluorescence. However, subtypes vary by which samples fluoresce: red blood cells, urine, teeth, gallstones, or feces.


The physical examination is facing extinction in modern medicine. The Top 10 Forgotten Diagnostic Procedures series was developed as a teaching tool for residents in family medicine to reaffirm the most important examination-based diagnostic procedures, once commonly used in everyday practice. For a complete pdf of the Top 10 Forgotten Diagnostic Procedures, go to


1. Asawanonda P, Taylor CR. Wood’s light in dermatology. Int J Dermatol. 1999;38(11):801–7. [PubMed]
2. Andrews MD, Burns M. Common tinea infections in children. Am Fam Physician. 2008;77(10):1415–20. [PubMed]
3. Puy H, Gouya L, Deybach JC. Porphyrias. Lancet. 2012;375(9718):924–37. [PubMed]
4. Ducharme EE, Silverberg NB. Selected applications of technology in the pediatric dermatology office. Semin Cutan Med Surg. 2008;27(1):94–100. [PubMed]

Articles from Canadian Family Physician are provided here courtesy of College of Family Physicians of Canada