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Picture this: You've just wrapped up a vacation in Mexico. You're relaxed, refreshed, and ready to kick it back home with your family and friends. With passport in hand, you approach customs, ready to breeze through, but wait — there's one more thing you have to do . . . Submit to an eye scan. 

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Yes, an eye scan. Think this is a sci-fi fantasy? Hardly. As of a December 10, this is a real thing happening to people passing the Otay Mesa and crossing into the U.S. through San Diego on foot. According to U.S. Customs and Border Protection, the eye scan program will run through June 2016.

The technology here isn't really new. It's part of a budding new field called biometrics — a type of security that uses physical traits to identify people. In fact, several educational institutions — including elementary schools and universities — have replaced the traditional ID card with iris scanners. James Hammond, head of Winthrop University's Information Technology department, said, "Iris scanning has a very high level of accuracy, and you don't have to touch anything. It can be hands-free security."

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But when it comes to our borders, it's estimated that 40 to 50 percent of the people illegally living in the U.S. crossed legally, but stayed long after their visas expired. U.S. border officials have been unable to keep track of who leaves Mexico by land — until now. And while the U.S. isn't intending to use this eye-scan technology to track these "overstayers," it could, further heightening immigrants' fears of being deported.

"But being able to track everyone is still a long way away," says Marc Rosenblum, deputy director of the U.S. Immigration Policy Program at the Migration Policy Institute. The effects on immigration, however, could be immediate. And that's definitely something to keep an eye on.