Wood's lamp - UV light for dermatologic assessment

see also:

• dermatology
• tinea fungal infections

• the Wood's lamp emits long wave UV light at 365nm which is used to detect flourescence of skin or hair to help diagnose various conditions
• it has various use cases:
  • differentiating epidermal vs dermal hyperpigmentation:
    • melanin acts as an absolute light absorber.
    • if a dark patch (like melasma) becomes significantly more prominent with sharp borders under the lamp, the pigment is superficial (epidermal).
    • if the patch remains unchanged, the pigment is settled deeper in the skin (dermal)
  • differentiating true vitiligo vs fading rash:
    • hypopigmented skin caused by scarring or poor blood flow stays dull.
    • true autoimmune vitiligo, which completely lacks pigment, glows with a stark, blinding chalk-white or blue-white outline
    • also useful for detecting ash-leaf vitiligo of tuberous sclerosis
  • differentiating acne vs Pityrosporum folliculitis:
    • red bumps on the chest or back can look identical to regular acne.
    • normal acne blocks follicles and shows minor red/orange speckles, whereas a fungal yeast overgrowth (Malassezia) will glow in broad yellow-green sheets
  • detecting tinea fungal infections
    • BUT cannot rule out a tinea infection on its own
    • common body ringworm and athlete's foot (typically caused by Trichophyton rubrum) generally show no fluorescence
    • Trichophyton tonsurans lives inside the hair shaft (endothrix) and does not fluoresce at all
  • assessing Rx success of tinea and detect potential areas for relapse
    • antifungal treatments destroy the fungi, the brightness of the fluorescence steadily diminishes
  • detecting Pseudomonas in wounds
  • detecting porphyrins in urine
  • detecting presence of dried cat urine
    • cat urine is generally highly concentrated and contains high concentrations of phosphorus and blood plasma proteins (such as albumin)
    • bright yellow or neon yellow-green fluorescence but over a long period, as the components break down, the glow slowly fades into a duller, pale white or faint yellow
    • BUT false positives due to:
      • carpet cleaners, wallpaper glue, laundry detergents (usually blinding, bright ice-blue or stark white rather than yellow), and spilled tonic water can also glow
    • thus need to check shape and smell to confirm
  • detecting scorpions at night (they are nocturnal):
    • scorpions fluoresce a vibrant, bright neon green, cyan, or blue-green colour
    • although you may need a higher wattage UV lamp for this