I was recently asked about a piece of equipment sometimes used in dermatology – the Wood’s light. This is something that most dermatology departments will have but rarely does it get a mention in day-to-day practice. So, what is it and what’s it good for?
What is Wood’s Light?
The Wood’s lamp took its name from an American Physicist, Dr Robert Wood who in 1903 invented a simple device for producing “black light” – what is effectively long wave UV radiation. Wood’s original device used a filter of barium silicate and nickel oxide which only permitted light in the range 320nm to 400nm to escape. The light emitted is low power and safe on the skin. Wavelengths just below this (290nm - 320nm) can produce burning of the skin.
The Wood’s light wavelength is just below the colour violet in the visible spectrum, so the light emitted may have this slight hue when in use. When applied to the skin it is absorbed and reflected as visible light in certain infectious and pigmentary changes. Normal skin does not fluoresce much at all but can produce a slight blue colour. Thickened skin may appear slightly yellow, dandruff white and fluff from clothes can also appear white! Anything applied to the skin can also produce fluorescence so before use it is important to ensure the skin is clean and free of any topical applications, make up or soap for example.
In clinical practice, it is used in a darkened room and shone onto the skin at a distance of about 25cms to elucidate any fluorescence which can be helpful is diagnosing certain skin conditions.
What fluoresces under Wood’s light?
For the podiatrist, it has a few uses. There is a misguided belief that it can be used to detect dermatophytes on the feet, but this is generally incorrect. The most common skin condition 0n the foot it can detect is erythrasma. Typically, an interdigital eruption of fungi and bacteria producing damp, malodourous web spaces. With a Wood’s light the condition will show up as a coral pink colour due to the presences of Corynebacterium in the web space. The condition has been covered in another blog on this website
The second use is the detection of Pseudomonas – typically found around and in chronic, damp wounds and as an occasional sub-ungual infection. The bacteria fluoresce in the presence of Wood’s light making the infection easier to detect. Thirdly, one of those strange things, but salicylic acid will also produce a fluorescence on the skin. I cannot think of a clinical application per se, but just maybe to confirm the cause of that suspicious, white soggy corn that sometimes presents in clinic that the patient denies ever putting anything on!
One other potential use of Wood’s light cited in the literature is for the treatment of warts, by the power of suggestion, in children. Caplan in 1967 writing in JAMA suggested this use.
Other things which fluoresce are probably less podiatric, but have uses in other disciplines of health care and crime detection:
Hypo- and hyper-pigmentation in the skin due to inflammation.
Head lice and scabies. In scabies a phosphorescent substance (fluorescein dye) is applied to the suspected area of infection and drawn up the burrow producing tell-tale lines diagnosing its presence.
Malassezia yeasts. Found in Pityriasis versicolor on the chest as orange-yellow colour. Hair follicle infection by the same organism can also produce a blue colour change.
Tinea capitis – certain fungi will fluoresce - mostly those involved in hair infection (mainly microsporum sp., which very occasionally occur on the feet)
Assessing the effectiveness of sunscreens applied to the skin – they appear black under Wood’s light highlighting the extent of coverage
Some ingested drugs may also cause a positive result such as tetracycline and mepacrine
Urine. This too can fluoresce, and the device is often sold as a “cat urine detector” on retail websites to find out where in your house your untrained feline has been busy! Human and canine urine will also show up to, so don’t be too quick to blame the cat.
Bank notes. Most currencies will fluoresce under UV light showing hidden text of the bank notes denomination or other designs not visible under normal daylight conditions.
Where can I buy one?
You can purchase these as medical devices from £20 upwards. My top tip is to look, not for a Wood’s light, but a “Cat Urine Detector” or “UV Blacklight torch”. These devices are sold more widely on the internet and cost a lot less! The cheapest one I could find was just £5.99 on Amazon. The only downside to the cheaper devices is their power. As they are lower wattage you would need to hold this closer to the subject probably a centimetre or two away instead of 20cms for the more powerful devices which run off the mains.