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A woman under black lights, her makeup glowing in vibrant hues. Powerofforever / Getty ImagesYou’ve likely spotted black lights at amusement parks, science museums, or Halloween displays. They may appear similar to regular fluorescent lamps or incandescent bulbs, but they operate in a completely different way. When switched on, they make white clothing, teeth, and other objects glow in the dark.
For instance, if you place a fluorescent poster in a dark room and shine a black light on it, the poster will light up in a brilliant glow. You may have also encountered paper that seems blank in regular light but reveals glowing words under black light. Many amusement parks use hand stamps that remain invisible until seen under these special lights.
In this article, we will uncover the science behind black lights, explore why they make some materials glow while leaving others unaffected, and dive into some fascinating uses of black light technology.
What Exactly Is a 'Black Light'?
When you switch on a black light bulb in a dark space, the bulb gives off a dim, purplish glow. What isn’t visible to the naked eye is the ultraviolet light emitted by the bulb.
Our eyes can detect light in a spectrum that spans from red to violet—this is the range of visible light. Beyond violet lies ultraviolet light, which is invisible to us. How Sunburns and Sun Tans Work explains ultraviolet light and its effects on our skin. A black light bulb produces UVA light, which is less harmful than UVB light.
Ultraviolet light exists just beyond violet in the light spectrum, and it is not visible to the human eye.The glowing effect you notice under a black light, whether it's on a neon-colored poster, an invisible stamp, or a freshly washed white T-shirt, is the result of phosphors at work.
A phosphor is a substance that gives off visible light when exposed to certain types of radiation. It takes the energy from UV light emitted by a black light and transforms it into visible light.
In the following section, we’ll explore how phosphors are utilized in standard fluorescent lighting and black lights.
White T-shirts and socks glow under black lights because many modern laundry detergents contain phosphors. These phosphors turn UV light into bright white light. This effect makes whites appear "whiter than white" in regular sunlight. What you're seeing in daylight is both the regular reflection of visible white light from the fabric, as well as the glow produced by the phosphors when they interact with UV light. In essence, your T-shirt is truly whiter than white!
Black Light Designs
Black lights are available in both tube and bulb configurations.A typical black light is simply a modified fluorescent lamp. Fluorescent lamps create light by passing electricity through a tube that contains an inert gas and a small amount of mercury. (For more details, check How Fluorescent Lamps Work.)
When powered up, mercury atoms release energy in the form of light photons. While some of these photons are visible, most are in the ultraviolet (UV) range. Since UV light waves cannot be seen by the human eye, fluorescent lamps must convert this energy into visible light using a phosphor coating on the tube's surface.
Phosphors are materials that emit light, or fluoresce, when exposed to light. When a photon strikes a phosphor atom, one of the phosphor’s electrons moves to a higher energy state, causing the atom to vibrate and generate heat. As the electron returns to its original state, it releases energy in the form of a photon with less energy than the incoming one, as some energy is lost as heat. In a fluorescent lamp, this emitted light falls within the visible spectrum, producing the white light we can see.
Black lights operate on the same fundamental principle. There are actually two types of black lights, but both function in nearly identical ways.
- A tube black light is essentially a fluorescent lamp with a specially designed phosphor coating. This coating absorbs harmful shortwave UV-B and UV-C radiation and emits UV-A light (similar to how the phosphor in a fluorescent lamp absorbs UV light and emits visible light). The dark glass tube blocks most visible light, allowing only safe, long-wave UV-A light, along with a bit of blue and violet visible light, to pass through.
- An incandescent black light bulb resembles a regular household light bulb but is equipped with filters that absorb most light emitted by the heated filament. These filters let only infrared and UV-A light (along with a little visible light) pass through.
In both types of black lights, the emitted UV light interacts with various external phosphors in the same way that the UV light inside a fluorescent lamp reacts with the phosphor coating. These external phosphors will glow as long as the UV light continues to shine on them.
In the upcoming section, we’ll explore objects that contain phosphors and examine some fascinating uses for black lights.
Black Light Uses
If you wandered around all night with a portable black light, you would quickly notice that phosphors are everywhere. Natural phosphors are present in your teeth, fingernails, and more. There are also many manmade phosphors, found in items like television screens, paints, fabrics, and plastics. Most fluorescent colored objects, such as highlighters, contain phosphors, and you'll see them in all kinds of glow-in-the-dark products. Clubs and amusement parks use special black light paints that shine in different colors. You can even find fluorescent black light bubbles, invisible black light ink, black light carpets, and fluorescent black light hair gel.
Regular highlighters serve as effective "black light pens." When placed under a black light, the fluorescent ink glows brightly!Besides making people and fluorescent posters look amazing, black lights have a range of practical uses. Here are a few examples:
- Appraisers use black lights to spot counterfeit antiques. Many modern paints include phosphors that glow under black light, unlike older paints that lack them.
- Technicians use black lights to track down hidden leaks in machinery. They inject fluorescent dye into the fuel supply and use the black light to illuminate it. For instance, a technician might detect a hidden air conditioner leak by adding fluorescent dye to the refrigerant. Black lights can also be used to spot counterfeit bills.
- Law enforcement officers utilize black lights to spot counterfeit money. The U.S. and many other countries have invisible fluorescent strips in their larger bills, visible only under a black light.
- Amusement parks and clubs use black lights to identify invisible hand stamps for re-entry.
- Forensic scientists use black lights to examine crime scenes. For instance, they may dust for fingerprints with fluorescent dye under black light, making the prints stand out from surrounding dirt. Black lights can also reveal bodily fluids like semen that naturally fluoresce.
Many of these uses, along with countless others, share a common theme: black lights make the invisible visible or highlight a specific substance amidst everything else. When you think about it, the possibilities for applying this phenomenon are endless!
If you're interested in learning more about black lights and various types of light energy, be sure to explore the helpful links provided below.
