<P> Black light is commonly used to authenticate oil paintings, antiques and banknotes . Black lights can be used to differentiate real currency from counterfeit notes because, in many countries, legal banknotes have fluorescent symbols on them that only show under a black light . In addition, the paper used for printing money does not contain any of the brightening agents which cause commercially available papers to fluoresce under black light . Both of these features make illegal notes easier to detect and more difficult to successfully counterfeit . The same security features can be applied to identification cards . </P> <P> Other security applications include the use of pens containing a fluorescent ink, generally with a soft tip, that can be used to "invisibly" mark items . If the objects that are so marked are subsequently stolen, a black light can be used to search for these security markings . At some theme parks, nightclubs and at other, day - long (or night - long) events, a fluorescent mark is rubber stamped onto the wrist of a guest who can then exercise the option of leaving and being able to return again without paying another admission fee . </P> <P> In medicine, the Wood's lamp is used to check for the characteristic fluorescence of certain dermatophytic fungi such as species of Microsporum which emit a yellow glow, or corynebacterium which have a red to orange color when viewed under a Wood's lamp . Such light is also used to detect the presence and extent of disorders that cause a loss of pigmentation, such as vitiligo . It can also be used to diagnose other fungal infections such as ringworm, microsporum canis, tinea versicolor; bacterial infections such erythrasma; other skin conditions including acne, scabies, alopecia, porphyria; as well as corneal scratches, foreign bodies in the eye, and blocked tear ducts . </P> <P> Fluorescent materials are also very widely used in numerous applications in molecular biology, often as "tags" which bind themselves to a substance of interest (for example, DNA), so allowing their visualization . Black light can also be used to see animal excreta such as urine and vomit that is not always visible to the naked eye . </P>

How long can a black light stay on