- published: 04 Aug 2015
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A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation (UV) in the long-wave (near ultraviolet, UVA) range, and little visible light. Other types of ultraviolet lamp emit large amounts of visible light along with the ultraviolet; however, a "black light" usually refers to a lamp that has a dark blue optical filtering material in the glass envelope of the bulb (or the lamp housing) which blocks most of the visible light, so the lamp emits mostly ultraviolet. Ultraviolet radiation is invisible, but a small fraction of visible light passes through the filtering material, with wavelengths no longer than 400-410 nm, and as a result, when operating the lamp emits a dim purple or violet glow. Wood's glass is one type of filtering material which is used in black lights.
Black light sources may be made from specially designed fluorescent lamps, mercury vapor lamps, light-emitting diodes, or incandescent lamps. In most black lights the blue optical filter material to block visible light is in the glass envelope of the light bulb, but in some types there is a separate filter glass in the lamp housing. In medicine, forensics, and some other scientific fields, such a light source is referred to as a Wood's lamp (named after Robert Williams Wood).