Laser tag venues and certain bowling alleys make liberal use of black lights because of the way they make neon colors glow. This glowing happens when the fluorescent light from the blacklight bulb reacts with the phosphors in or on a given object. While the resulting glow is intrinsically cool and frequently used in decorating, the ability to detect phosphors also makes blacklights practical for a number of things.
Find Fake Antiques
If you suspect an antique is a forgery, or might have been repainted a little more recently than an antique dealer claims, a good way to put this to the test is to shine a blacklight on the situation. If the paint on the object in question glows, it is not an antique. Blacklights only glow when they shine on phosphors. Modern paint contains phosphors. As such, anything that glows under a blacklight is probably not an antique, and anything the glows under a blacklight in certain places was touched up recently.
Detect Counterfeit Money
Here's a clever use, brought to you by the US Mint. Every official US treasury note has a strip of phosphors on it that's invisible to the naked eye but immediately obvious under a blacklight. So, if you put a 20-dollar bill under a blacklight and see no strip, the bill is a fake. While the phosphor strip is not impossible to replicate, but most counterfeiters don't bother.
Write hidden messages
Write with a highlighter on your hand and you probably won't see it. Put that hand under a blacklight and it will appear clear as day. Many amusement parks and concert venues take advantage of this for re-admission---upon entry you are stamped with an invisible phosphor stamp, which is exposed by a blacklight when you attempt to re-enter. While not immediately practical for those outside the intelligence community, writing secret messages to be viewed under a blacklight is a great way to demonstrate to children how blacklights work and is also just plain fun. Try it out with an identically colored highlighter and piece of paper.
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