Wood's lamp - UV light for dermatologic assessment

see also:

• dermatology
• tinea fungal infections

• the Wood's lamp emits long wave UV light (typically in the 320–400 nm range, with a peak wavelength around 365 nm and traditionally use Wood’s glass - barium silicate with nickel oxide that blocks most visible light while transmitting UVA, but new devices usually just use 365nm LEDs) which is used to detect flourescence of skin or hair to help diagnose various conditions
• 365 nm wavelength is energetic enough to excite many endogenous and exogenous fluorophores in skin, hair, nails, and some materials, causing them to fluoresce in visible light, yet it remains in the safer UVA range
• 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