Black lights may look just like normal fluorescent lamps or incandescent light bulbs , but they do something completely different. Switch one on, and white clothes, teeth and various other things glow in the dark. You may have also seen pieces of paper that look blank in regular light but spell out a glowing message under a black light. In this article, we'll find out exactly what's going on here. We'll also see why black lights make some objects glow but not others, and we'll look at some interesting black light applications. If you turn on a black light bulb in a dark room, what you can see from the bulb is a purplish glow.
Black Lights and Ultraviolet Lamps
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A blacklight or often black light , also referred to as a UV-A light , Wood's lamp , or ultraviolet light , is a lamp that emits long-wave UV-A ultraviolet light and very little visible light. One type of lamp has a violet filter material, either on the bulb or in a separate glass filter in the lamp housing, which blocks most visible light and allows through UV,  so the lamp has a dim violet glow when operating. A second type of lamp produces ultraviolet but does not have the filter material, so it produces more visible light and has a blue color when operating. Blacklight sources may be specially designed fluorescent lamps , mercury-vapor lamps , light-emitting diodes LEDs , lasers , or incandescent lamps ; although incandescents produce almost no blacklight except slightly more for halogen types , and so are not considered true blacklight sources. Although many other types of lamp emit ultraviolet light with visible light, black lights are essential when UV-A light without visible light is needed, particularly in observing fluorescence ,   the colored glow that many substances emit when exposed to UV. Black lights are employed for decorative and artistic lighting effects, diagnostic and therapeutic uses in medicine,  the detection of substances tagged with fluorescent dyes , rock-hunting , the detection of counterfeit money , the curing of plastic resins, attracting insects  and the detection of refrigerant leaks affecting refrigerators and air conditioning systems. Strong sources of long-wave ultraviolet light are used in tanning beds. Fluorescent black light tubes are typically made in the same fashion as normal fluorescent tubes except that a phosphor that emits UVA light instead of visible white light is used. The type most commonly used for black lights, designated blacklight blue or "BLB" by the industry, has a dark blue filter coating on the tube, which filters out most visible light, so that fluorescence effects can be observed. These tubes have a dim violet glow when operating.