Why do my patches light up under a black light?
Because the light is ultraviolet, and skin that has lost its pigment gives some of it back as a glow you can see. DermNet describes what shows up: skin with reduced pigment fluoresces bright blue-white, and sometimes yellowish green. Its borders look sharper under that light too. Evidence Evidence Evidence
Why this matters
Fluorescence is a coloured glow. Something absorbs the black light and emits it again at a longer wavelength, in the part of the spectrum your eye can read. DermNet puts the blue-white down to accumulated biopterins. MedlinePlus supplies the contrast that makes it obvious in a room: normally, skin does not shine under ultraviolet light at all.
Questions for your dermatologist
- Would a Wood lamp exam show anything about my patches that an ordinary look does not?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Is it really the same light my dermatologist uses?
The same family of light, yes. The National Library of Medicine lists Black light test as an alternative name for a Wood lamp examination. DermNet goes the other way and lists nightclubs among the users of a Wood lamp, for checking fluorescent re-entry stamps. Evidence Evidence Evidence
Why this matters
DermNet gives the physics. A traditional Wood lamp is a low-output mercury arc behind a filter of barium silicate and nickel oxide. It emits 320 to 450 nm, peaking at 365 nm, and Robert W. Wood invented it in 1903. A 2023 systematic review of vitiligo diagnostics gives the same figures. Two records agree on them, so this page states them rather than crediting one. Modern black lights may instead be BLB fluorescent tubes, mercury vapour lamps, light-emitting diodes or incandescent bulbs. The fluorescent tubes carry a dark blue coating that removes most of the visible light. That is why the fitting itself reads as violet in a dark room while the working part of its light stays invisible.
Is that light doing anything to my skin?
DermNet says the black light a Wood lamp emits is harmless, and its reason is specific rather than general. In DermNet’s words, the lamp does not emit the short-wavelength ultraviolet B at 290 to 320 nm. So, DermNet says, it does not cause sunburn or otherwise damage healthy skin. Evidence Evidence Evidence
Why this matters
The glow itself leaves nothing behind. Light goes in and comes back out at a longer wavelength, so it stops the moment the fitting does. DermNet records one exception. Somebody with extreme photosensitivity might develop a rash on skin a black light reached. It adds that the examination is usually very brief, and unlikely to cause problems even then. MedlinePlus lists a single risk, and it is not the skin: do not look directly into the ultraviolet light, because the light may harm the eye.
Questions for your dermatologist
- Is there any reason I should avoid ultraviolet lighting that other people would not?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Am I the only thing lighting up in there?
No, and it is worth knowing. DermNet says fabric, topical medicines and soap residue fluoresce too, and that clothing lint often shines bright white. MedlinePlus adds some deodorants, make-up and soaps to the same list. Evidence Evidence
Why this matters
In a clinic those are a nuisance, because DermNet says they cause false positives. That is why you may be asked not to wash the skin or put anything on it before an exam. In a room they mean something else. A white shirt, a stripe of detergent somebody missed and a set of teeth are all lit up in the same light. The contrast on your skin is one of several things in the room doing it.
- Clubs, bars and music venues, including the re-entry stamp on the back of your hand.
- Bowling alleys, mini golf, laser tag and arcades.
- Concerts and festivals, and school or college events.
- Some theme park rides, photo booths and escape rooms.
If I cover my patches, does the makeup light up too?
It can. Both records name make-up among the things that fluoresce under this light, alongside moisturiser, deodorant and soap residue. DermNet says skin should ideally carry none of them before an exam, precisely because they cause false positives. Evidence Evidence
Why this matters
That is a real answer to something people ask quietly. If you use camouflage or self-tanner, a black light is not a neutral test of how well it matches. Neither record tested a named product, and neither says which cosmetics glow and which do not. What they establish is that the category can.
Questions for your dermatologist
- Would you expect anything I put on my skin to change how it looks under a Wood lamp?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Evidence and update context
This optional layer shows the evidence boundary reviewed for this page as of 2026-08-22.
- What this evidence supports
- Fluorescence is a coloured glow. Something absorbs the black light and emits it again at a longer wavelength, in the part of the spectrum your eye can read. DermNet puts the blue-white down to accumulated biopterins. MedlinePlus supplies the contrast that makes it obvious in a room: normally, skin does not shine under ultraviolet light at all.
- What it does not establish
- All three records describe a darkened examination room, not a room lit for a night out. None measured how bright a patch looks under a venue light fitting. None grades how visible you are to anybody else. What the glow comes from is not settled either. The biopterins are DermNet’s attribution. The systematic review never names them, and puts the prominence down to the missing pigment that would otherwise sit over it. How much this shows on your skin is the bigger open question. DermNet reports the vitiligo use as identifying affected areas in light-skinned people. The systematic review is blunter. It grades false negative results in darker skin tones as a limit of the method itself.
Applicability check
Who was studied?
These fields show what the cited evidence reports about the people and body sites behind outcome or safety claims. “Not reported” means the reviewed source omitted the field; “Pending exact source review” means SteadySkin has not made that determination yet.
DermNet says the black light a Wood lamp emits is harmless, and its reason is specific rather than general. In DermNet’s words, the lamp does not emit the short-wavelength ultraviolet B at 290 to 320 nm. So, DermNet says, it does not cause sunburn or otherwise damage healthy skin.
- Age range
- Pending exact source review
- Condition subtype
- Pending exact source review
- Severity or extent
- Pending exact source review
- Sample size
- Pending exact source review
- Geography and care setting
- Pending exact source review
- Skin tone or phototype
- Pending exact source review
- Race
- Pending exact source review
- Ethnicity
- Pending exact source review
- Body sites
- Pending exact source review
What that means for this page: This is explicitly gated as a safety claim, but exact population extraction is still pending. It cannot publish or support an estimate of individual risk until that review is complete.
Evidence behind this page
Sources
Each evidence badge opens the source and its limits. The full list stays available here.
- DermNet (A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand)Patient education · Patient education, tier 5Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that a Wood lamp examination is a diagnostic test. Skin or hair is examined while exposed to the black light a Wood lamp emits. It supports that black light is invisible to the naked eye. It sits in the ultraviolet spectrum, at a wavelength just shorter than the colour violet. It supports that the lamp glows violet in a dark environment. That is because it also emits some light in the violet part of the spectrum. It supports that a traditional Wood lamp is a low-output mercury arc covered by a Wood filter. That filter is barium silicate and 9% nickel oxide. It supports that the lamp emits wavelength 320 to 450 nm, with a peak at 365 nm. It supports that a Baltimore physicist, Robert W. Wood, invented it in 1903. It supports that modern black light sources may be specially designed BLB fluorescent lamps. They may also be mercury vapour lamps, light-emitting diodes or incandescent lamps. It supports that fluorescent black light tubes carry a dark blue filter coating. That coating filters out most visible light. It supports that fluorescence is a coloured glow. It is seen when certain substances absorb black light and emit it again at a longer wavelength in the visible spectrum. It names collagen and porphyrins as such substances. It supports that items on the skin surface can also fluoresce. It names fabric, topical medications and soap residue. It supports that skin to be examined should ideally not have been recently washed. It should ideally not have had makeup, deodorant or moisturising cream applied. Those can fluoresce and cause a false positive result. It supports that room lights are turned off and the surroundings darkened completely. It supports that the examiner waits to adapt to the dark. The lamp is then held about 10 to 30 cm away and the skin examined for a few seconds. It states that the examination is painless and safe. It supports that normal healthy skin is slightly blue under the lamp. It shows white spots where skin is thickened, yellow where it is oily and purple where it is dehydrated. It supports that clothing lint often shines bright white. It supports that a Wood lamp is used to identify the extent of pigmented or depigmented patches, and to detect fluorescence. It supports that loss of pigmentation, giving vitiligo as its example, is one of the things a Wood lamp may reveal. It reports that use as identifying affected areas in light-skinned people. It supports that hypopigmented skin has sharper borders under black light. It supports that hypopigmented skin fluoresces bright blue-white, or sometimes yellowish green, due to accumulated biopterins. It supports that the black light emitted by a Wood lamp is harmless. It supports that the lamp does not emit short-wavelength ultraviolet B radiation at 290 to 320 nm. So it does not cause sunburn or otherwise damage healthy skin. It supports that a patient with extreme photosensitivity might develop a rash on skin exposed to black light. It adds that the examination is usually very brief, and unlikely to cause problems even in very photosensitive patients. It supports that it is prudent to ask the patient to close their eyes when the face is examined. It supports that nightclubs use a Wood lamp to check fluorescent re-entry stamps. That sits in a list of non-medical uses that also names law enforcement, banks and gemology.
What it does not support
It is a dermatology reference page, not a trial and not a measurement of any one person. Its statement that black light is harmless describes a medical Wood lamp used briefly at 10 to 30 cm. It measures no venue light fitting and no evening of exposure. It sets no safe length of time under any lamp. That statement also rests on the absence of ultraviolet B, which is what causes sunburn. It is not a finding about skin where pigment has been lost. It reports the vitiligo use as identifying affected areas in light-skinned people. It grades nothing about how visible a patch is to another person. It names no lighting product sold for a venue and measures no output from one. It puts ultraviolet B at 290 to 320 nm, where the World Health Organization guide registered here puts it at 280 to 315 nm. Nothing on it identifies a rash on a reader or directs the care of one. It says nothing about a course of narrowband UVB prescribed by a clinician. Its author line is dated August 2014 and the page records a last review of 11 July 2023, so parts of it are older than the revision date registered here.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.
- MedlinePlus, U.S. National Library of MedicinePatient education · Patient education, tier 5Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that a Wood lamp examination is a test that uses ultraviolet light to look at the skin closely. It supports that Black light test and Ultraviolet light test are alternative names for the same examination. It supports that the test is usually done in a dermatologist’s office, and that you sit in a dark room for it. It supports that the doctor holds the lamp 4 to 5 inches, or 10 to 12.5 cm, from the skin to look for colour changes. It supports that you will have no discomfort during the test. It supports that no special steps are needed beforehand, beyond a doctor’s instruction about creams or medicines on the skin. It supports that the test is done to look for bacterial infections, fungal infections and porphyria. It supports that it is also done to look for skin colouring changes, naming vitiligo and some skin cancers. It supports that not all types of bacteria and fungi show up under the light. It supports that normally the skin will not shine under the ultraviolet light. It supports that the exam may help a doctor confirm a fungal or bacterial infection, or diagnose vitiligo. It supports that it may also help a doctor learn what is causing light or dark coloured spots on skin. It supports that washing the skin before the test may cause a false-negative result. It supports that a room that is not dark enough can change the result. It supports that other materials glow under the light. It names some deodorants, make-up, soaps and sometimes lint. It supports that the risk it lists is to the eye. It says not to look directly into the ultraviolet light, as the light may harm the eye.
What it does not support
It is a consumer medical encyclopedia entry, not a trial. It describes a brief examination in a clinician’s office. It measures nothing about a light fitting in a public place, and gives no length of time that is safe under any lamp. It publishes no wavelength and never explains why skin without pigment looks different under the light. It measures nothing about how visible a patch is to another person. It does not diagnose vitiligo on its own, and says only that the exam may help a doctor do so. It lists no risk to skin at all, which is an absence of a statement rather than a finding of safety. Its content is written and reviewed by A.D.A.M. for the National Library of Medicine, and its stated review date of 14 October 2024 is what fixes its currency.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.
- Frontiers in Medicine (Abdi P, Anthony MR, Farkouh C, Chan AR, Kooner A, Qureshi S, Maibach H)Systematic review · Clinical research, tier 2Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that a Wood lamp is a handheld device. It emits long-wave ultraviolet light. It supports a range of 320 to 450 nm, with a peak at 365 nm. It supports that the lamp carries a magnifying lens. It supports that patches outlined under a Wood lamp were much more prominent. That led to finding new patches not seen in ordinary light. It also led to finding that known patches had spread. It supports that the review grades the method as low cost, painless and easy to use. It grades it as rapid, portable and non-radiative, with results in real time. It supports that the review grades four limits. The first is false negative results, above all in darker skin tones. The second is that the result depends on the operator. The third is that lighting can limit it. The fourth is that uneven skin colour makes the spread of pale skin hard to judge. It supports one further point, made in the review’s section on ultraviolet light photography. Vitiligo shows up well in those photographs because the skin fluoresces more. That is because the pigment that would sit over it is gone.
What it does not support
It is a systematic review of ways to measure and diagnose vitiligo in people who already have it. It is not a trial. It measures nothing on any one reader. Its Wood lamp material sits inside the section on ultraviolet light photography. There is no separate Wood lamp section. Its sentence about fluorescence and missing pigment is written about photography. So it is not a Wood lamp finding. It is not a statement about what in the skin makes the glow. It never names biopterins and does not settle that question. It describes a clinical exam. It measures no venue light fitting. It sets no length of time that is safe under any lamp. It carries no finding about harm to skin or eyes from a black light. So it supports no part of a safety conclusion. It measures nothing about how visible a patch is to another person. Its false negative limit in darker skin tones is a limit of the method. It is not a prediction about any reader.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.