If you're watching a National Football League game on television or in person, see if you can spot a green dot on the helmet of one of the players. It's usually the quarterback or a linebacker—players responsible for calling plays.
That green dot indicates he is a player with a tiny two-way radio installed in his helmet to communicate with his coach. It's how coaches call in the play, rather than the old system of shuffling in a player—usually a receiver or running back on offense—to deliver the play directly to the quarterback.
As you might imagine, those green-dotted helmets are closely guarded by the NFL and its teams to ensure they don't fall into the wrong hands.
"Coaches communicate from the press box to the field on a wired system, and on the field there are 10 coaches who have wireless belt packs," said Bill Lipscomb, president of WBL Service, a Seattle company associated with the NFL for more than 30 years. Lipscomb is managing telecommunications and internet infrastructure for Super Bowl XLIX in Glendale, Ariz.
“These wireless belt packs are how the coaches communicate between each other,” he said. “And there are two coaches, one offense coach and one defense coach, who have the ability to talk to one player on the field so they can call in the plays.”
That player, by rule, must have a green dot on his helmet to mark him as the "wired" player.
"So during the game the NFL will be looking from their command center to see if they have more than one green dot," Lipscomb said.
"The green-dotted helmets remain under lock and key and are brought out by league officials before the game," he said. "They are secured immediately post game."
So why all the secrecy? "To make all the teams comfortable and make sure the equity rule has been met," Lipscomb said.
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