Few things in sport spark as many arguments as a VAR decision. Video technology now decides goals, penalties and offsides in top football leagues worldwide, from the English Premier League to the FIFA World Cup. Here is the engineering behind the screens.
Goal-line technology: was it over the line?
A goal counts only when the whole ball crosses the whole goal line. With the ball moving fast and players in the way, the eye often cannot tell. Goal-line technology answers it automatically.
- Camera-based systems use several high-speed cameras around each goal to track the ball in three dimensions, similar to cricket's ball-tracking.
- When the ball fully crosses the line, the system sends a signal to the referee's watch, typically within about a second.
VAR: the video assistant referee
VAR is a team of officials watching the match on multiple camera feeds in a video operations room. They can review only clear and obvious errors or serious missed incidents in four situations:
The VAR team advises the referee, who can accept the advice or review the footage on a pitchside monitor. The final decision always rests with the on-field referee.
Semi-automated offside
Offside decisions can come down to centimetres. Semi-automated offside technology, used at major tournaments, tracks many points on each player's body using special cameras under the stadium roof, while a sensor inside the ball detects the exact moment it is kicked. Software calculates the offside line and alerts the video officials, who confirm the decision and produce a 3D animation for fans.
Why VAR is still controversial
- Frame rates: even fast broadcast cameras capture discrete moments, so the exact kick instant may fall between frames.
- Subjective calls: technology can show what happened, but judging a handball or foul still needs human interpretation.
- Delays: long reviews interrupt the flow and the joy of celebrating a goal.
Cricket faces similar trade-offs. See how our guide to cricket's DRS handles uncertainty with "umpire's call".
Frequently asked questions
Can VAR check every decision?
No. VAR is limited to the four match-changing situations, and only clear and obvious errors are meant to be overturned.
Do players wear sensors?
Some leagues use player-tracking for performance data, but semi-automated offside relies mainly on optical tracking from cameras plus a sensor in the ball.