Wood's light is light in the UVA1 part of the spectrum near 365 nanometers. This ultraviolet light can cause fluorescence in Corynebacterium minutissimum (erythrasma) and in various fungi and yeast infections.
Pityrosporum species, the cause of tinea versicolor, show a yellow fluorescence as a fine line around the edge of the typical scaling patch.
Microsporum audouinii and canis, both of which can invade hair, fluoresce a brilliant green under Wood's light.
Microsporum distortum, gypseum, and nanum can induce fluorescence.
Trichophyton schoenleinii causes a pale green fluorescence of infected hair.
Wood's light is an important tool in the management of scalp ringworm, particularly epidemic ringworm in schools. It permits the rapid survey of large populations. It is important to examine the skin or hair in a very dark room. A few minutes of adjustment are required for the observer to be able to see the fluorescence after darkening the room.
Lint and some creams and ointments can cause bright fluorescence in lesions and this should be clearly distinguished from the disease specific fluorescence patterns.
Urine in patients with porphyria cutanea tarda fluoresces orange.
Wood's light examination can be quite valuable in evaluating patients with hyper and hypopigmentary disorders and in determining the extent of some melanomas with indistinct margins such as the lentigo maligna melanoma.