A black light is a lamp that emits electromagnetic radiation almost exclusively in the soft near ultraviolet range that is only partially visible. It is mainly seen by humans using the low-light receptors in the eye, which are sensitive to near ultraviolet. The low-light receptors however also have the distinctive feature that they are more accurate in the peripheral vision, this means black light will always look out of focus when looked at directly. In medicine, forensics, and some other scientific fields, such a light source is referred to as a Wood’s lamp (named for American physicist, Robert Williams Wood).
Black light sources have many uses. They may be employed for decorative and artistic lighting effects, for diagnostic and therapeutic uses in medicine, for the eradication of microorganisms, for the observation or detection of substances tagged with other substances that exhibit a fluorescent effect, for the curing of plastic resins and for attracting insects. Strong sources of long-wave ultraviolet light are used in tanning beds. Black light lamps are used for the detection of counterfeit money.
The Daily Omnivore 
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