Black light flashlights can have entertaining uses. They highlight the phosphors in your teeth, clothes and in certain colors and paints, causing them to glow brightly.
Forgery and counterfeit money can be discovered using a black light flashlight. Shining the light on a U.S. bill will reveal an invisible stripe across it. Shining a light on a forged antique will reveal phosphors in the paint that would not have been present if the piece was real.
Certain body fluids glow under black light. Police forensic teams use black light to reveal evidence at crime scenes.
Bars and amusement parks use black light flashlights to view hand stamps for readmission or to show that the patron is not underage. The flashlight proves that the hand stamp was not forged.
Florescent dyes that glow under a black light flashlight can be used in machinery to reveal hidden leaks. This application is commonly used in detecting holes in refrigerant tanks to fix car air conditioners.