by David DiSalvo

In the battle to control the high seas, there’s a new ray of light—the modern laser.

In 1918, as the Great War was nearing its end, eccentric genius Nikola Tesla announced that he’d successfully harnessed the power of focused, amplified light. With the new technology, he planned to invent a death ray, capable of killing thousands of people with one blast. But Tesla was ahead of his time; when he tried to sell the concept to a handful of governments, they turned him down.

Although the death ray remains a science fiction fantasy, lasers—devices that amplify highly focused beams of light—are quite real. Of course, today’s lasers are rarely used to attack people. Instead, they’re usually enlisted as defensive weapons to help control crowds, safeguard military checkpoints, and, most recently, protect ships from pirates. The trick to their protective powers? The ability to safely (and temporarily) blind people.

Modern lasers really aren’t much different than those cheap laser pointers you might use during a presentation. If you’ve ever looked directly into the beam, you’ve probably noticed that your eyes start to hurt after a second, and when you look away, you see a floating spot. That’s because when the laser beam reaches your eye, it sends a lot of light into a small area of your retina, which overloads your optical circuits and makes you see spots.

Lasers that are used as defensive weapons work the same way, only with a bigger, more intense beam. These devices come in two flavors: a 250-milliwatt version the size of a flashlight, with a 30-yard range, and a 500-milliwatt version that resembles a pistol, with a range of up to 1.5 miles. (By comparison, those classroom laser pointers use about 5 milliwatts.) Both emit a light so intense that it will temporarily blind anyone looking into it. The blindness is called the “dazzling effect,” which is why manufacturer Laser Energetics trademarked the device the “Dazer Laser.”

Dazed and Diffused

If you’re a police officer trying to control a riot, the dazer can be your best friend. Unlike tasers, dazers can repel multiple people at a time, and they don’t require physical contact. The blindness they cause lasts for up to 15 minutes, while other unpleasant side effects, such as nausea and vomiting, may linger for more than an hour. The end result is that a rowdy crowd will find it difficult to walk, let alone fight—at least for a while.

What works for police officers trying to control crowds also works for military personnel at checkpoints in Iraq and Afghanistan. When an individual or group approaches a checkpoint, the guards usually ask them to stop. If they refuse, the military version of a dazer laser keeps intruders at bay until reinforcements arrive. Neither tinted car windows nor sunglasses reduce the device’s effectiveness. This beats the more aggressive alternatives, and it gives the military time to determine if the threat is genuine.

Similarly, for cargo ships at sea, dazers are a godsend. We’ve all heard of the infamous Somali pirates in the Gulf of Aden, but piracy is much more widespread. In fact, the number of pirate attacks around the globe roughly doubles every year. Any ships large enough to carry valuable cargo, including privately owned “super yachts,” are at risk.

In 2009, Scott Buchter, CEO of the Finnish corporation Lasersec, introduced a type of dazer specifically for super yachts called the SeaLase. The device is 10 times as powerful as a crowd-control dazer, and it looks like an elongated telescope. Unlike water cannons and other defensive weapons built for ships, SeaLase is effective at distances exceeding 2.5 miles. Pirates can see the light from miles away, signaling to them not to come any closer. But if they continue to approach the vessel, the dazer’s effects increase from mild disorientation all the way to flash blindness, a more severe degree of dazzling that lasts longer.

Although SeaLase costs around $100,000 per unit, super-yacht owners are snatching them up like hotcakes. “We’re selling these units as fast as we can make them,” said Buchter. His company is also in negotiations to develop defensive lasers for oil rigs on land and at sea.

Meanwhile, the U.S. Air Force is building larger defensive lasers that could be deployed from the air, the water, or the ground. These military-grade lasers are about 1 billion times as powerful as a handheld laser pointer. They’re meant to incinerate their targets, namely enemy missiles, from several miles away. They could be used to protect vulnerable sites, such as nuclear power plants and water treatment facilities, from terrorist attacks. But in theory, the laser could also be used as a massive death ray, capable of killing thousands of people with one blast. Tesla would be proud.

This article originally appeared in mental_floss magazine. Wouldn’t a subscription make a great gift for someone super special on your list?