Black lights emit long wave ultraviolet (UV) light that allows materials and substances invisible to the human eye under normal lighting to fluoresce and produce visible color and light. Black lights are most known as novelty items used for lighting blacklight posters and fluorescent objects or for blacklight parties with glow-in-the-dark paint, the color of which is only visible under black lights. However, black lights are a useful and common tool used for everyday purposes, from crime scene forensics to hand stamping at nightclubs.
Crime Scene Inspection
Forensic scientists use black lights to find hidden evidence at crime scenes. Black lights can reveal human blood, urine and semen, even if they are cleaned up and invisible to the human eye under normal lighting. Forensic scientists can also find and examine fingerprints under black light after dusting the prints with a chemical that fluoresces under black light.
Black lights are a useful tool when checking how sanitary a hotel room or public restroom is, as a black light will reveal any urine, semen, feces and saliva hiding out in the last places you'd want to find them. Although some hotels use black lights to ensure high standards when it comes to sanitation, it's always smart to carry a small black light flashlight when traveling to make sure your hotel room is clean, since not all hotels take this measure to ensure cleanliness.
Pet Stain Detection and Pest Control
Just as human body secretions are visible under a black light, so are the urine, feces and other body secretions of animals. Black lights are useful for finding the unknown location of pet urine odors and for finding evidence of rats or mice lurking in your home. If you live in an area where scorpions are common, you can use a black light to search your home for hidden scorpions, the hard outer shells of which contain fluorescent chemicals that glow under a black light. Black lights are also used in bug zappers to attract bugs before they are killed with an electric shock when they fly into the trap.
Counterfeit Detection and ID Verification
Anyone who handles money on a regular basis can use a black light to detect counterfeit bills. All legitimate currency have a unique vertical authenticity strip, the color and placement of which is unique to the value of the bill. These authenticity strips contain fluorescent dyes for illumination and luminescent solids for a specific color that only become visible under a black light. Black lights are also used for ID verification, as most credit cards, driver's licenses and passports contain a UV watermark that is only visible under a black light.
Antique and Art Inspection
Black lights are often used in antique and art inspection for authentication and repair tests. Antique inspectors use black lights to determine whether a painted object is actually an antique, a touched-up antique, a reproduction or a newly painted object, as modern paints contain phosphors that glow under a black light while older paints do not. Any repairs made to antiques also become visible under black lights, helping antique inspectors to determine the extent and quality of repairs. Black lights are used in the same way for art inspection to determine the age of paintings and other art works and to find repairs, touch-ups, damage and imperfections.
A/C and Other Liquid Systems Leak Detection
Mechanics and air conditioning professionals use black lights to determine coolant and oil leaks in air conditioners and other liquid systems. To determine leaks, a liquid fluorescent dye is injected into the leaking system, allowing the location of the leak to become visible under a black light.
Hand Stamp Screening
UV hand stamps that are only visible under a black light are sometimes used to screen permitted guests at clubs, parties and other events where entry or re-entry is monitored. Unlike visible stamps, UV stamps will not leave a visible mark or stain and are harder to duplicate or transfer.