Black lights look just like normal fluorescent lamps, but they do something completely different. When you turn on your black light to attract insects, you see that your white clothes, teeth and various other things glow in the dark, while the bulb itself only emits a faint bluish light.
Beside entomological collecting equipment, black lights are all around us, in bars and clubs, science museums, amusement parks and teenagers' bedrooms, among other places, but to most of us, they are a total mystery.
The conventional black light design is just a fluorescent lamp with a couple of important modifications. Fluorescent lamps generate light by passing electricity through a tube filled with inert gas and a trace amount of mercury.
When energized, mercury atoms emit energy in the form of light photons. They emit some visible light photons, but mostly they emit photons in the ultraviolet (UV) wavelength range. UV light waves are too short for us to see as they are completely invisible. Fluorescent lamps must convert this energy into visible light. Bulb manufactures do this with a phosphor coating around the inside of the glass tube.
Phosphors are substances that give off light or fluoresce when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level, causing the atom to vibrate and create heat. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum, the phosphor gives off white light we can see.
Black lights work on this same principle.
The emitted UV light from the black light bulb reacts with various external phosphors in exactly the same way as the UV light inside a fluorescent bulb reacts with the phosphor coating. The external phosphors glow as long as the UV light is shining on them. If you walked around all night with a portable black light, you would discover that there are phosphors all over the place. There are lots of natural phosphors, in your teeth and fingernails, among other things. There are also a lot of phosphors in man-made material, including television screens and some paints, fabric and plastics. Most fluorescent colored things, such as highlighters, contain phosphors, and you'll find them in all glow-in-the-dark products. Clubs and amusement parks use special black light paint that glows different colors. You can also buy fluorescent black light bubbles, invisible black light ink, fluorescent black light carpet and even fluorescent black light hair gel.
The first thing most people notice when you switch on a black light is that some of their clothing glows. This is because most laundry detergents contain phosphors to make whites appear brighter in sunlight. Sunlight contains UV light that makes the whites glow "brighter than white." Dark clothes don't glow because the dark pigments absorb the UV light.
In addition to attracting insects, making people and fluorescent posters look cool, black lights have some practical applications. For example:
Art appraisers and antique dealers use black lights to detect forgeries of antiques. Many paints today contain phosphors that will glow under a black light, while most older paints do not. Law enforcement officers can use black light to identify counterfeit money. The United States and many other countries include an invisible fluorescent strip in their larger bills that can only be revealed when exposed to black light.
Repairmen use them to find invisible leaks in machinery by injecting a little fluorescent dye into the fluid or gas line and illuminate the line with a black light. For example, they might detect an invisible Freon or Puron leak in an air conditioning system by adding fluorescent dye to the refrigerant. Amusement parks and clubs use them to identify invisible fluorescent hand stamps for readmission.
Forensic scientists use black lights to analyze crime scenes. Fingerprints can be found when they dust with a fluorescent dye under a black light. This makes it easier to pick the fingerprints out from surrounding dirt. Black lights can also identify semen and other bodily fluids that naturally fluoresce.
Most of these uses have common denominator, black lights make the invisible visible or to isolate one specific substance from everything else around it.