The Wood’s lamp is a diagnostic tool used in medicine, particularly dermatology, that emits long-wave ultraviolet (UV-A) light. This instrument focuses UV light, often peaking at 365 nanometers, through a specialized filter known as a Wood’s filter. The underlying principle is fluorescence, where compounds on or within the skin absorb this UV radiation. They then re-emit the energy as visible light at a longer wavelength, creating a distinct glow. This fluorescence allows observers to identify changes not visible under ordinary light.
How the Wood’s Lamp Examination Works
The Wood’s lamp examination is a simple, non-invasive process requiring specific preparation for accurate results. The room must be completely darkened so fainter fluorescent colors are clearly visible. The light source, which emits UV-A light peaking at 365 nm, should be allowed to warm up for about a minute before use.
The clinician holds the lamp approximately 10 to 30 centimeters away from the area being examined. Patient preparation is important, as makeup, lotions, or soaps can contain substances that fluoresce, potentially leading to a false-positive reading. The specific color and pattern of the resulting fluorescence is the diagnostic signal. Normal healthy skin typically appears a slight blue color or does not fluoresce significantly.
The Specific Meaning of Blue-White Fluorescence
The appearance of a bright blue-white or stark white fluorescence is a significant finding, often pointing toward two distinct categories of conditions. The most recognized cause is depigmentation, specifically associated with the autoimmune condition Vitiligo. In Vitiligo, skin areas that have lost their melanin appear strikingly bright white or blue-white under the UV light.
The fluorescence in Vitiligo results from UV light being absorbed and re-emitted by biopterins within the depigmented keratinocytes. This intense contrast allows clinicians to precisely determine the margins of patches, often revealing subclinical lesions not visible under normal lighting. A similar, pale blue-white fluorescence can also be observed in other pigmentary disorders, such as the hypopigmented ash-leaf macules associated with Tuberous Sclerosis.
Another specific condition producing a bluish-white glow is Malassezia folliculitis, a yeast infection of the hair follicles. While Tinea Versicolor (caused by the same yeast) typically fluoresces yellow-green or coppery-orange on the skin surface, the organism infecting the hair follicle emits a distinct bluish-white light. This variation is related to the metabolic byproducts of the Malassezia yeast in the confined follicle environment.
It is necessary to differentiate true biological fluorescence from external factors during the examination. Areas of naturally thickened skin (hyperkeratosis) often reflect the light as white spots, which is reflection, not true fluorescence. Common contaminants like clothing lint, soap residue, or makeup can also emit a bright white or blue-white glow. These false positives must be accounted for, but the distinct, sharp borders seen in true depigmentation provide a valuable clue.
Other Diagnostic Colors Seen Under the Lamp
The blue-white color is one of several diagnostic hues that can appear under the Wood’s lamp, each pointing to a different underlying condition.
- Coral Red: Highly suggestive of the bacterial infection Erythrasma. This color is produced by Corynebacterium minutissimum, which synthesizes coproporphyrin III that accumulates in the superficial skin layer. The intense red glow serves as a rapid diagnostic marker for this common skin fold infection, which often mimics fungal rashes.
- Green or Bright Green: Often associated with fungal infections of the scalp, known as Tinea capitis. Microsporum species produce this color due to pteridine compounds in the infected hair shafts. The fluorescence is confined to the hair itself, distinguishing it from other scalp conditions. Separately, the bacterium Pseudomonas aeruginosa also generates a green fluorescence.
- Yellow-Green or Coppery-Orange: Typically displayed by Tinea Versicolor, caused by the Malassezia yeast. This distinct color is attributed to pityrialactone, a porphyrin metabolite produced by the yeast, and is often found on the chest or back.
- Orange-Red: Caused by the acne bacteria, Cutibacterium acnes, which makes pilosebaceous follicles fluoresce. This finding provides a visual map of the bacteria’s colonization in the pores, allowing clinicians to assess the condition’s distribution and severity.
Clinical Applications of Wood’s Lamp Diagnosis
Beyond providing an initial diagnosis, the Wood’s lamp examination has several practical applications in clinical dermatology. It serves as an immediate, non-invasive method to confirm a suspected infection, providing results faster than laboratory cultures. The presence or absence of the characteristic fluorescent color can immediately validate a clinical suspicion, streamlining the diagnostic process for conditions like Erythrasma or Tinea capitis.
The tool is also valuable for monitoring the effectiveness of treatment over time. Once medication eliminates the causative organism, the fluorescent byproducts cease to be produced, and the glow disappears. This change provides a clear visual indication that the therapy is working, guiding the clinician on when to discontinue treatment.
Furthermore, the Wood’s lamp is instrumental in defining the precise boundaries of lesions. In conditions like Vitiligo or Tinea Versicolor, the UV light enhances the contrast between affected and unaffected skin, allowing for accurate mapping of the lesion’s full extent. This capability is useful for surgical planning or ensuring that topical medications are applied to the entire affected area. It can also help distinguish the depth of hyperpigmentation, such as in Melasma, by showing if the pigment is concentrated in the superficial epidermis (enhanced) or the deeper dermis (unchanged).