For the diagnosis of what skin conditions is Wood’s light useful. What features are typically seen on exposing these conditions to Wood’s light?

HPA report on the diagnosis, management and prevention of tinea capitis in the United Kingdom (1) states:

“Filtered ultraviolet (Wood’s) light elicits a green fluorescence from some dermatophyte fungi, mainly Microsporum species, in hair infections. Exposure to Wood’s light is a useful screening procedure for taking specimens from Microsporum infections. Equally it is unhelpful in many of the anthropophilic infections seen in the UK as they do not fluoresce.”

A 2004 Finnish guideline on psoriasis states

“Erythrasma is a macular brown area with few symptoms, most often found in the armpits or groin. It is caused by overgrowth of diphtheroids of the normal skin flora. These areas fluoresce coral pink under long-wave ultraviolet radiation (Wood's light).”

A CKS guideline on Fungal (dermatophyte) skin infections (3) states:

“What is the role of Wood's light examination?
'Wood's light' examination (i.e. using a filtered ultraviolet lamp) is not necessary for routine diagnosis of fungal skin infections in the community setting.
Certain zoophilic infections, such as those caused by Microsporum canis and M audouinii, cause the hair to fluoresce bright green.
Most of the current infections in the UK are caused by different dermatophytes and are negative under Wood's light examination [Higgins et al, 2000].

Wood's light examination may be useful in specific circumstances:
Screening close contacts of infected individuals.
Determining the extent of infection.
Identifying areas for microbiological sampling.
Evaluating treatment response.
Differentiating between erythrasma (caused by the bacterium Cornebacterium, which fluoresces coral-red under Wood's light) and ringworm infection (which does not usually fluoresce).
A Merck Manual chapter on diagnostic tests (4) states

“Wood's light: Wood's light (black light) can help distinguish hypopigmentation from depigmentation (depigmentation of vitiligo fluoresces ivory-white and hypopigmented lesions do not). Erythrasma fluoresces bright orange-red. Tinea capitis caused by Microsporum canis and Microsporum audouinii fluoresces a light, bright green. (Note: Most tinea capitis in the US is caused by Trichophyton species, which do not fluoresce.) The earliest clue to cutaneous Pseudomonas infection (eg, in burns) may be green fluorescence.

Another Merck Manual chapter on Tinea Versicolor (5) notes that Wood's light examination reveals golden-white fluorescence. Yet another chapter on hyperpigmentation (6) discusses melasma and notes “epidermal pigmentation accentuates on Wood's light or can be diagnosed with biopsy.”

A search in Medline yielded a paper which describes some further uses of the Wood’s light (7).

“We demonstrate the utility of the Wood's light in a practice that specializes in the evaluation of pigmented lesions. The Wood's light assisted the physician in locating the site of a completely regressed primary cutaneous melanoma, determining the clinical borders of a lentigo maligna melanoma, differentiating between agminated naevi and a naevus spilus and detecting the recurrence of pigmentation after the excision of a dysplastic naevus, and also proved useful in monitoring a large segmental speckled atypical lentiginous naevus for change.”

One other review was found with no abstract, the full paper may give further guidance (8).

7. Paraskevas LR, Halpern AC, Marghoob AA Utility of the Wood's light: five cases from a pigmented lesion clinic. Br J Dermatol. 2005 May;152(5):1039-44
8. Asawanonda P, Taylor CR.Wood's light in dermatology.