In an enormous grocery store in northern France, the lights above the aisles aren’t all they seem to be. They look ordinary—more than a mile and a half of fixtures exuding bright light, folded into a grid overhead—but they’re actually flickering faster than the human eye can see. The unique patterns each individual section of lighting emits are a 21st-century twist on Morse code, meant not for people, but for the cameras on their phones.

If shoppers grant the store’s app access to their smartphone’s front-facing lens, the phone can watch for the lights and use the pulses to pinpoint its location. Doing so allows the app to plot the best routes for shopping lists, tracking people as they travel through the store. (The guidance might come in especially handy for first-time visitors to the 84,000-square foot Carrefour “hypermarket,” the French equivalent of a Walmart.)

Location information is one of the most valuable types of data a retailer can gather from its customers, says Joseph Turow, a professor of communications at the University of Pennsylvania. (I interviewed Turow about the future of retail surveillance last month.) If a retailer knows where you spent most of your time inside of a store, it can follow up with discounts for a product you looked at but didn’t buy—either after you’ve left the store, to encourage a return trip, or even right as you’re lingering in the aisle, to nudge you to buy it now. In the U.S., Target and Walmart are rumored to use lighting technology to locate smartphone-toting shoppers, but aren’t forthcoming about their plans.

The system being tested in the French supermarket offers a glimpse of what light bulbs might soon be able to do. There, the light fixtures’ imperceptible blinking is transmitting a very basic stream of data, but LED light bulbs are capable of sending a lot more than just a simple pattern. They can deliver an entire internet connection.

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Five years ago, Harald Haas, a professor of electronics engineering at the University of Edinburgh, was pacing on a small stage. He was giving a TED talk as high-definition video of blooming flowers looped behind him. He approached a desk lamp he’d placed on a gray tabletop, and stuck his hand underneath the light. The video stopped. When he removed his hand, it started up again.

The light, he explained, was transmitting video data to a receiver beneath the lamp, at a speed comparable to Wi-Fi. In a lab setting, light bulbs can beam data at a speed of more than 10 gigabits per second—that’s really, really fast.

After his 2011 talk, Haas started a company called pureLiFi, which announced this year that it will install light-based internet in the Paris headquarters of the French real-estate development company Sogeprom.

“We’re basically at the stage that Wi-Fi was 15 years ago,” Haas told me. For now, connecting to a Li-Fi network requires a USB dongle—a throwback to the age of Wi-Fi cards that stick out of the side of your laptop.

Data carried over visible light doesn’t need a direct sightline between a light bulb and a device in order to work. Just like ambient light in a room still illuminates the floor beneath a table, data-laden light beams can reflect off of walls and objects on their way to a device. Haas envisions that the laptops of the future will have multiple light sensors—maybe one at every corner of the screen—so that they can continue receiving uninterrupted even if one or more of the receivers is covered. (References to LiFi have been found buried in the code of recent versions of iOS, but the technology doesn’t exist in any smartphone or computer yet.)

Unlike a Wi-Fi signal, visible light can’t, of course, travel through walls or floors—but that can actually be a useful feature, Haas says. In sensitive environments like a government building, administrators can confine a network only to the rooms where people need to access it, avoiding the threat of a hacker intruding into a wireless network that’s bled across the hall or out onto the street. Visible light can also be used to transmit data where Wi-Fi networks can interfere with delicate electronics, like in hospitals.

One of the draws of light-delivered internet is that the lighting has already been installed in just about every office and residential building. The hard part is connecting the light bulbs to the net: If each one has to be wired individually, the ease of using light bulbs is canceled out; if each fixture still has to use Wi-Fi to communicate with a central router, then LiFi will never be faster than Wi-Fi itself.

Haas suggests that power-line communication can help connect light fixtures by sending data over electric lines in a home or an office. But there might be another way to connect light bulbs to the internet without using cables or Wi-Fi.

A group of researchers at the Swiss Federal Institute of Technology in Zurich and Disney Research, led by Stefan Schmid, proposed a system called EnLighting that functions as a mesh network: It allows each light bulb to “see” signals blinked out by nearby bulbs and repeat them. That way, only one light bulb needs to be connected to the internet—the others can pick up the signal it’s putting out and rebroadcast it.

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Even if LED lights become a standard way to send data, they come with one enormous drawback: Turning off the lights would cut off the internet.

To solve that problem, Zhao Tian, a Ph.D. candidate at Dartmouth, designed a system that can transmit data with such dim and spaced-out pulses that active light bulbs appear to be off. If there’s any ambient light at all—sun filtered through a window, or a computer or TV screen—Tian’s “DarkLight” LED looks totally extinguished; only in complete darkness does it emit a faint glow, which Tian describes as “even lower than the moon at night.”

Lowering the power and speed at which the LED flickers means transmitting information much more slowly. But the system can calibrate itself to the ambient light around it, so it only slows down and dims itself when it’s in darkness.

With Wi-Fi and cellular data speeds getting faster by the year, a new way to send data wirelessly may not seem absolutely necessary. But research firms forecast that the market for Li-Fi could be worth more than $100 billion by 2022, perhaps betting that it could leapfrog Wi-Fi’s speed limitations. Given that Apple already appears to be exploring the technology for its phones, we may someday soon be holding our devices up to the light instead of waving them toward the router to get a better signal.