A Wood's light is a hand-held fluorescent lamp emitting "long wave" ultraviolet A. In dark conditions, certain infections fluoresce on exposure to the light.
- Fungal infection of the hair and scalp due to the cat ringworm Microsporum canis fluoresces green.
- Pityriasis versicolor, which mainly affects the trunk, is due to certain species of malassezia. It fluoresces yellow-green when the infection is active, providing the patient has not washed prior to examination (the responsible chemical is water soluble).
- Bacterial infection in skin flexures due to erythrasma Corynebacterium minutissimum fluoresces coral-pink.
As colour contrast increases on exposure to a Wood's light, the extent of pigmentary disorders can be determined.
- Vitiligo (white patches anywhere)
- Pityriasis versicolor (pale patches mainly on the trunk)
- Melasma (brown patches on the face)
Characteristic urinary fluorescence is also helpful in the diagnosis of hepatic porphyrias (porphyria cutanea tarda).