Blacklight

A blacklight, also called a UV-A light, Wood's lamp, or ultraviolet light, is a special lamp that gives off long-wave ultraviolet light and very little visible light. One kind of blacklight has a violet filter that blocks most of the normal light, letting only the ultraviolet light shine through. This makes the lamp look like a dim violet glow. These lamps are called "blacklight blue" or BLB. Another type of blacklight does not have this filter, so it looks blue and is often used in bug zapper insect traps.

Blacklight fluorescent tubes. The violet glow of a blacklight is not the UV light itself, but visible light that escapes being filtered out by the filter material in the glass envelope.

Blacklights can be made using different technologies, such as special fluorescent lamps, mercury-vapor lamps, light-emitting diodes (LEDs), lasers, or incandescent lamps. In medicine and forensics, they are called Wood's lamps after Robert Williams Wood, who invented the original Wood's glass UV filters.

Blacklights are very useful because they can show fluorescence, a colored glow that some materials give off when exposed to ultraviolet light. People use blacklights for many purposes, such as creating decorative lighting, helping doctors diagnose problems, finding substances marked with fluorescent dyes, hunting for rocks and scorpions, spotting counterfeit money, curing certain plastics, attracting insects, and finding leaks in refrigerators and air conditioning systems. Strong ultraviolet lights are also used in tanning beds.

Medical hazard

UV-A light can be bad for your eyes and skin, especially if you are very close to strong lights. It can make your skin tan and may make it age faster or get wrinkled. UV-A might also help cause some kinds of skin cancer. It can affect your eyes, both right away and over a long time. The good news is that most blacklight products for home use have wavelengths above 350 nanometres and are usually safe to use normally.

Types

Fluorescent

Fluorescent blacklight tubes work like regular fluorescent tubes, but they have a special coating inside that creates ultraviolet A (UVA) light instead of white light. The most common type, called blacklight blue or "BLB," has a dark blue filter that blocks most visible light. This lets us see fluorescence effects clearly, and these tubes glow with a dim violet light when they are on. They are different from "blacklight" or "BL" tubes, which don’t have this filter and appear brighter blue. These brighter tubes are used in bug zappers where visible light doesn’t matter.

Compact fluorescent blacklight bulb

Incandescent

Blacklights can also be made from regular incandescent bulbs by adding a UV filter coating, like Wood’s glass, to the bulb. This was how the first blacklights were made. However, incandescent bulbs are not very efficient at creating UV light because most of their light is visible light, which needs to be blocked. Because of this, these bulbs get very hot and don’t last as long as other types.

Mercury vapor

Spectrum of a blacklight fluorescent tube. FWHM spectral bandwidth of the 370 nm peak is about 20 nm. The tiny secondary peak (2) is light from the mercury vapor line at 404 nm leaking through the filter, which gives the lamp its purple glow.

High-power mercury vapor blacklight lamps range from 100 to 1,000 watts. They don’t use coatings but rely on the natural UV light produced by mercury gas under high pressure. These lamps have special glass that blocks visible light and harmful short-wavelength UV light. They are mainly used for theater and concert displays because they are good at producing UVA light.

LED

Some LEDs can create ultraviolet light, but it is usually limited to wavelengths longer than 380 nm. The light produced this way is often mixed with visible light, so it’s not as strong for certain uses.

100-watt incandescent blacklight bulb

Bug zappers

Various phosphor compositions used in blacklight
Phosphor Mixture	Peak (nm)	Width (nm)	Philips suffix	Osram suffix	U.S. Type	Typical use
—	450	50	—	/71	—	hyperbilirubinaemia, polymerization
SrP ₂O ₇:Eu	420	30	/03	/72	—	photochemical polymerization
SrB ₄O ₇:Eu	370	20	/08	/73	("BLB")	forensics, lapidary, night clubs
SrB ₄O ₇:Eu	370	20	—	/78	("BY")	insect attraction, polymerization, psoriasis, tanning beds
BaSi ₂O ₅:Pb	350	40	/09	/79	"BL"	insect attraction, tanning beds
BaSi ₂O ₅:Pb	350	40	/08	—	"BLB"	dermatology, lapidary, forensics, night clubs
SrAl ₁₁O ₁₈:Ce	340	30	—	—	—	photochemistry
MgSrAl ₁₀O ₁₇:Ce	310	40	—	—	—	medical applications, polymerization

Safety

Blacklights give off a type of light called UVA. This is the gentlest kind of ultraviolet light. It does not burn the skin, but it can still affect it over time. It may make skin age faster and cause wrinkles.

UVA light can also affect the DNA inside skin cells. It does this in a less direct way than stronger UV light, and can reach deeper into the skin. However, the light from blacklights is usually too weak to cause serious damage.

UVA light can also affect the eyes, both quickly and over a long time.

Uses

Ultraviolet radiation is invisible to our eyes, but when certain materials are lit up with it, they glow in different colors. This glowing is called fluorescence, and blacklights are needed to see it because other ultraviolet lamps give off too much visible light.

Fluorescein glowing under ultraviolet light

Blacklights have many uses. In hospitals, doctors use a special lamp called a Wood's lamp to check for skin problems. They shine ultraviolet light on the skin and watch for glowing that can show signs of disease.

Blacklights are also used to find security marks on things like money and passports. These marks glow under a blacklight, helping to prove that items are real. They can also be used to find hidden messages or marks.