“Black lights”, sometimes simply referred to as “UV lights”, are similar in shape to fluorescent tubes. They are used commonly in night clubs because certain paints, dyes, and clothing “glow” when exposed to them. The “glow” is actually an effect called fluorescence, wherein a UV-sensitive pigment absorbs UV light and emits different light which humans can readily see. Black lights emit a small amount of light in the visible spectrum, violet light above 400nm, that humans can see which is why the bulbs appear to glow violet when turned on. The majority of their light is invisible to humans.
Light may be referred to by its wavelength, measured in nanometers (nm), which is 10-6 meters (very small). Visible light, detectable by the human eye, is between 380nm and 760nm. Ultraviolet light is broken up into three categories:
UVA light extends from 320nm to 400nm. UVB light extends from 280nm to 320nm. UVC light is below 280nm. UVC light is also called “germicidal” light because of its intense energy which destroys living creatures (as we know them). The Earth’s atmosphere blocks the sun’s UVC light from reaching the earth, allowing life as we know it to exist.
Therapeutic UV phototherapy utilizes UVA or UVB spectrum light.
UVA is commonly administered with a drug called psoralen, which sensitizes the skin to the UVA light increasing the effect of treatment. This type of treatment is called PUVA (the “P” standing for psoralen). PUVA has a long history of being very effective at treating a variety of skin disorders. PUVA treatment has been linked to skin cancers after many treatments, the incidence increasing steeply after 250 treatments.
UVB is commonly administered by itself, but may also be administered in combination with other medications, for example coal tar (e.g. Goeckerman regimen).
Narrow-band UVB (NB-UVB) has been shown to be more effective than UVB, and as effective as PUVA.
In studies evaluating specific UVB wavelengths and their therapeutic value versus erythema (reddening of the skin), it was shown that:
- Erythema peaked at a UVB light wavelength of 297nm continuing downwards towards 280nm (minimum of UVB range), fell off sharply above 300nm, and was the least nearing 310nm up to 320nm
- This means that for the same dosage of UVB light, much more erythema, (sun-burn like side effects) are produced in the skin.
- As targeted UV phototherapy is based around delivering very high dosages of UV to diseased tissue, light nearing 310nm peak wavelength can be delivered in far higher dosages than light at 297nm because side effects are less near 310nm.
- These findings prompted the UV light industry to introduce “Narrow-Band” UVB lamps, or NB-UVB lamps, which emitted UVB light primarily between 300 and 320nm, with a peak near 310nm.