Wood's light (WL) is a rapid, cost-effective, accessible, and non-invasive diagnostic method based on the use of an ultraviolet (UV) radiation source with a wavelength of approximately 365nm. Since its invention in 1903 by physicist Robert Wood, it has facilitated the diagnosis of multiple skin diseases.1 It has traditionally been used for superficial fungal infections and pigmentation disorders. In recent years, the use of WL seems to have decreased among dermatologists.2 In this article, we will review the utility of Wood's light in inflammatory dermatoses, such as infectious conditions, and in the management of skin cancer.

Originally, WL consisted of a mercury lamp with a barium silicate filter containing nickel oxide, allowing UV radiation with wavelengths from 320 up to 400 nanometers (nm), with a peak at 365nm.1 Classic Wood's lamps are associated with a 1.5× magnifying lens. However, they can be bulky and expensive, which may hinder their use.3 Currently, LED blacklight flashlights are available, which are lightweight, small, and with emission peaks from 365 to 395nm and prices around €15 to €30. These have also shown to be effective for detecting fluorescence, though they lack a magnification lens.4–6 Recently, UV light with a 365nm wavelength (UV365) has been added to some dermatoscopes, allowing for more detailed descriptions of fluorescence patterns in various skin diseases.7

The diagnostic capabilities of WL are based on the phenomenon of fluorescence. UV365 photons excite electrons in molecules called fluorophores, which, upon their return to their ground state, release photons in the visible light range.8 At WL wavelengths, the endogenous fluorescence of the skin mainly originates in the dermis, where cross-links of pepsin and collagenase in structural collagen emit a bluish fluorescence.8,9 However, melanin efficiently absorbs this wavelength, reducing the fluorescence intensity. This allows WL to highlight differences between hypo- or hyperpigmentations.10,11 WL also reveals the accumulation of exogenous fluorophores, such as substances produced by fungal and bacterial infections, or endogenous ones, as in the case of porphyrias (Table 1).12

WL should be used in a dark room.1 It was traditionally recommended to turn on the lamp 60s before the examination, as mercury lamps do not emit the full spectrum of radiation until a sufficiently high pressure is reached.12 With LED lamps, however, this time lag is not necessary. The light source should be held 10–12cm away from the skin,1 although with LED lamps that have more focused light, it may be necessary to move them 30 or 40cm away. In the case of suspected infections, washing the skin beforehand should be avoided, as it may dilute the fluorophores and produce false negatives.1 However, in pigmentation disorders or pigmented lesions, it is preferable to wash the face and remove cosmetics and sunscreens, as these can distort the image and complicate the clinical delineation of lesions.13 Hyperkeratotic scales and secretions such as saliva, serum, semen, and milk13 are some of the causes of false positives, as well as exogenous elements, such as colored markers, laundry detergents, lint, lemon juice, cosmetics, dyes, and ointments.13

Chronic exposure to UVA radiation, present in WL, is associated with the development of cataracts and ocular aging.14,15 However, ophthalmologists have indicated that WL has no negative ocular effects.16 It is recommended to cover the eyes of children, as their lenses lack the protective pigment found in adults, which absorbs UVA radiation, allowing it to reach the retina.14,15,17 Additionally, children tend to look directly into the light.

In the examination of lesions of porokeratosis with WL, the “diamond necklace” sign has been described, corresponding to the white fluorescence of the hyperkeratotic scale surrounding the bluish-black center6,18 (Fig. 1A and B). However, this fluorescence is inconsistent.6 In follicular porokeratosis, a pattern of white, dotted fluorescence is observed in the central area, corresponding to the lamellae of dilated follicles.6 Under WL, subclinical lesions of morphea appear as well-demarcated dark macules, which can facilitate early diagnosis and follow-up of these patients.19

Figure 1.

(A) Disseminated actinic porokeratosis. (B) Wood's light: “Diamond collar” effect, white fluorescence of the hyperkeratotic scale. (C) Incipient facial vitiligo. (D) A notable increase in visibility of hypopigmented areas under Wood's light.

(0.64MB).

The classic use of WL is in pigmentation disorders, such as vitiligo.16,20,21 The absence of melanin allows the visualization of the bluish fluorescence of the dermis with a very well-demarcated border.10,11 Under dermoscopy with UV365 light, homogeneous follicular fluorescence has been reported in 40% of vitiligo cases.7 WL can detect subclinical lesions, enabling early diagnosis and evaluation of treatment response21–23 (Fig. 1C and D). In tuberous sclerosis, WL can highlight hypomelanotic macules, especially confetti depigmentation, which is less apparent under visible light than the classic lanceolate macules.24–26 In melasma, WL can help identify the depth of melanin deposition and potentially predict treatment response27–29 (Fig. 2). In epidermal melasma, hyperpigmentation darkens under WL29–31. In contrast, dermal melasma does not show increased contrast.29–31 However, the histological correlation of melasma classification by WL is controversial: some studies suggest good correlation, while others indicate that all melasmas have a dermal component.27,28,32

Figure 2.

(A) Facial melasma (cheek). (B) Enhancement of hyperpigmented areas under Wood's light. (C) Facial melasma (cheeks and upper lip). (D) Hyperpigmented areas under Wood's light.

(0.37MB).

Progressive macular hypomelanosis is a pigmentation disorder caused by Cutibacterium acnes, a gram-positive bacterium that resides in the hair follicle and produces coproporphyrin III.33–36 WL accentuates the hypopigmented areas and shows red fluorescence in the follicles of the hypopigmented zones33–35 (Fig. 3A and B). WL allows differentiation from pityriasis versicolor, which presents yellowish fluorescence (Fig. 3C and D); from pityriasis alba, which does not show fluorescence due to irregular parakeratosis; or from post-inflammatory hypopigmentation and idiopathic guttate hypomelanosis, which show the bluish fluorescence of the dermis.20,35

Figure 3.

(A) Progressive macular hypomelanosis. (B) Red fluorescence in the follicles of the hypopigmented areas under Wood's light (easier to see in person than in the photograph). (C) Clinically subtle pityriasis versicolor. (D) Yellow fluorescence under Wood's light.

(0.48MB).

Erythrasma, caused by Corynebacterium minutissimum, presents coral-red fluorescence due to the production of coproporphyrin III37,38 (Fig. 4). This allows differentiation from other causes of intertrigo that do not show fluorescence, such as irritative intertrigo, candidiasis, or inverse psoriasis.37,38 In the case of tinea cruris, up to 25% may exhibit blue-green fluorescence if caused by Microsporum, which produces the fluorophore pteridine.7,16 Trichobacteriosis, caused by Corynebacterium flavescens, shows white-yellow fluorescence attached to the axillary hair,37,39,40 whose fluorophore responsible remains unknown to this date.16,41Pityriasis versicolor displays yellow-green fluorescence, originating from the pityriellactone porphyrin produced by Malassezia globosa.42-44 In tinea capitis caused by Microsporum canis, blue fluorescence is observed due to the fluorophore pteridine.45-48 Infections by Trichophyton generally do not present fluorescence, with the exception of Trichophyton schoenleinii, which causes favus tinea and shows pale blue fluorescence.16,20 Infections by Pseudomonas aeruginosa present green fluorescence, due to pyoverdine, an iron-chelating pigment.49-51 The use of WL in wounds with suspected superinfection can allow early diagnosis of superinfection by this pathogen, as well as in nail infections (“green nail”)51,52 (Fig. 5A).

Figure 4.

(A) Inguinal erythrasma. (B) Coral-red fluorescence of under Wood's light. (C) Interdigital erythrasma on the left foot. Coral-red fluorescence under Wood's light.

(0.2MB).

Figure 5.

(A) Green nail syndrome caused by Pseudomonas aeruginosa. (B) Scabies burrows under Wood's light (white arrows). (C) Dermoscopic image of the scabies burrows (white arrows).

(0.15MB).

In scabies, WL reveals bluish-white fluorescence in the acarine burrow53,54 (Fig. 5B and C). If the burrow is evaluated with a dermatoscope with UV365 light, a white or green fluorescence point corresponding to the mite's body can be seen.53,54

In 2012, the Canadian Family Physician journal ranked WL as number one in the top 10 forgotten diagnostic procedures.75 Although some studies suggest its use has increased over time, most refer to its underuse in the routine clinical practice.76,77 A survey conducted in Andalusia (Spain) showed that only 42.5% of dermatologists had WL and used it; 26% had it but did not use it, and 33% did not have it available.2

WL represents a fundamental tool in the diagnostic arsenal of dermatology. It is a quick, cost-effective technique with a short learning curve. It can highlight specific characteristics of pigmentary disorders, as well as inflammatory, infectious, and parasitic dermatoses, providing valuable information for diagnosis and therapeutic planning. Additionally, it may assist in the delineation of surgical margins for ML and non-melanoma skin cancer, although the evidence is still limited and, in some cases, contradictory.

None declared.