How Lightbulbs Watch You Buy Groceries

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.