What a Wood's lamp actually shows
A £200 ultraviolet torch is the cheapest instrument in this clinic and it routinely changes a treatment plan by several thousand pounds. Here is what it is doing.
There is a black torch on the wall of Room 1 that costs less than a single session of anything else we do. It emits ultraviolet at around 365 nanometres, we turn the lights off, and for about ninety seconds we look at your face in the dark.
Patients occasionally find this the least impressive part of the consultation. It is frequently the most consequential.
Why ultraviolet separates depth
Melanin absorbs ultraviolet strongly. Pigment sitting high in the epidermis therefore absorbs the lamp’s output and reads as a sharply darker patch with a defined edge — the contrast against surrounding skin actually increases under the lamp.
Pigment that has dropped into the dermis is sitting under a layer of tissue that scatters ultraviolet before it arrives. Less light reaches the melanin, less is absorbed, and the patch barely changes. Its edges stay soft.
So: sharpens under the lamp, mostly epidermal. Does not, mostly dermal. Most real faces are a mixture, and what we are really recording is the ratio.
Why the ratio decides the money
Epidermal pigment responds well to topicals and gentle layered peels. It is treatable, it is comparatively cheap, and the results hold if the sunscreen holds.
Dermal pigment does not lift meaningfully with any of that, and — importantly — it does not lift safely with heat either, because the melanocyte producing it is reactive and responds to thermal insult by producing more. A patient with predominantly dermal melasma who is sold a laser course will usually look better at week six and worse at month five.
What it does not tell you
The lamp is not a diagnosis on its own. It does not distinguish melasma from post-inflammatory hyperpigmentation — history does that. It fluoresces certain bacteria and fungi, which is useful but a different job. It is not reliable on very deep skin tones, where the contrast it depends on is reduced and dermoscopy carries more of the weight.
It is a two-minute test that tells you which of two expensive paths not to walk down. Ask whether you had one.