Hawk-Eye is built from a network of 10 cameras around the court that capture 60 high-resolution images per second. At least five cameras cover every bounce of the ball.
A centralised computer system rapidly processes the images, triangulates the position of the ball and calculates a flight path – the yellow streak you see behind the ball in the Hawk-eye graphics.
Hawk-Eye collects data for every shot taken in the match, not just the close calls.
This is to make sure all 10 cameras are doing their job – as well as providing plenty of talking points for analysts and pundits.
The 26-strong Hawk-Eye team ensure the cameras are calibrated to the court lines, and must do this again when faded lines are repainted throughout the tournament.
During games, a team of four work from a commentary booth: two Hawk-Eye team members, one big-screen operator and one review official (a certified umpire).
How many times can players use Hawk-Eye?
Players are given unlimited opportunities to challenge a call in each set at Wimbledon.
However, once three incorrect challenges are made, that player can’t challenge again until the next set.
If the set goes to a tiebreak, an extra challenge is awarded to each player.
It rarely happens, but the umpire can reject a challenge if it is an “unreasonable request or that it was not made in a timely manner”, according to the International Tennis Federation rules.
When was Hawk-Eye introduced to Wimbledon?
After it was first tested in 2004, the system was implemented on Centre and Court 1 in 2007.
It is now used across Centre Court plus courts 1,2,3,12 and 18.
Players on courts without Hawk-eye have to solely rely on line umpires to make the right calls.
How accurate is Hawk-Eye?
The Hawk-Eye system has a 2.2mm margin of error, with some research claiming the system can be as much as 10mm off.
Why? The ball may move too quickly to be properly captured on camera as all cameras have a finite frame-speed.
As one University of Cardiff paper says, “if the frame-speed is, say, 100 frames per second, and the ball is moving at about 100 mph it will travel about 1.5 feet between frames”.
This margin of error can cause controversy during big points.
During the 2007 Wimbledon final between Rafael Nadal and Roger Federer, a ball that appeared to be out was called in by 1mm.
There are other occasional problems with the system. During a 2017 match between Rafael Nadal and Borna Coric, the Hawk-Eye graphic failed to show on screen, with Nadal appearing to be sceptical about the final call.
But despite the problems with Hawk-Eye, it’s a lot more reliable than human line judges.
Studies have suggested 8.2% of all line calls involving balls within 100mm of a court line will be called incorrectly by line judges.
This means you can expect line judges to make four errors per set, and Hawk-Eye will happily mop up the mistakes as long as the technology is present on court.
Caroline Wozniacki’s third round defeat to Zhang Shuai was shrouded in controversy after three calls went against the Danish due to Hawk-Eye, though she insisted the system was inaccurate in furious outbursts at the umpire.