Black lights, also referred to as a UV-A light, Wood's light, or simply ultraviolet light, is a lamp that emits long wave (UV-A) ultraviolet light and not much visible light. The lamp has a violet filter material, either on the bulb or in a separate glass filter in the lamp housing or a UV LED (light emitting diode), which blocks most visible light and allows through the UV wave length of light, so the lamp has a dim violet glow when operating. Black light bulbs which have this filter have a lighting industry designation that includes the letters "BLB".
Black light sources may be specially designed fluorescent lamps, mercury vapor lamps, light-emitting diodes, lasers, or incandescent lamps. 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).
Black lights are essential when 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, and attracting insects. Strong sources of long-wave ultraviolet light are used in tanning beds. Although the low power UV-A emitted by black lights is not a hazard to skin or eyes and can be viewed without protection
Fluorescent Black Lights
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 operation
Mercury Vapor Black Light
High power mercury vapor black light lamps are made in power ratings of 100 to 1000 watts. These do not use phosphors, but rely on the intensified and slightly broadened 350–375 nm spectral line of mercury from high pressure discharge at between 5 and 10 standard atmospheres (500 and 1,000 kPa), depending upon the specific type. These lamps use envelopes of Wood's glass or similar optical filter coatings to block out all the visible light and also the short wavelength (UVC) lines of mercury at 184.4 and 253.7 nm, which are harmful to the eyes and skin. A few other spectral lines, falling within the pass band of the Wood's glass between 300 and 400 nm, contribute to the output. Mercury vapor lamps are used mainly for theatrical purposes and concert displays. They are more efficient UVA producers per unit of power consumption than fluorescent tubes.
LED Black Lights
Ultraviolet light can be generated by some light-emitting diodes, but wavelengths below 380 nm are uncommon and the emission peaks are broad, so only the very lowest energy UV photons are emitted, within predominantly visible light. Also used for nightclubs, theatrical purposes and concert displays