Badminton line calls at top level work in three layers: a line judge watches each line and calls “out” or signals “in”; the umpire can overrule a line judge who has clearly got it wrong; and at tournaments with an Instant Review System (Hawk-Eye), players can challenge a call and multi-camera tracking decides. A single phone camera can’t match that for very close landings, so an honest system says “too close to call” instead of guessing.
Who calls what in professional badminton
The BWF Laws of Badminton (Law 17) split the job between technical officials:
- Line judges sit on chairs looking straight down their assigned lines, ideally 2.5–3.5 m back from the court, according to the BWF Instructions to Technical Officials. For “out” they call loudly and extend both arms horizontally; for “in” they say nothing and point at the line with the right hand; if unsighted, they cover their eyes with both hands.
- The service judge sits on a low chair at the net post opposite the umpire and calls service faults by the server, including the rule that the whole shuttle must be below 1.15 m when struck (see serve rules).
- The umpire runs the match from the high chair. A line judge’s decision normally stands, but under Law 17.5.1 the umpire must overrule if it is “beyond reasonable doubt” that the line judge “has clearly made a wrong call”. If an official is unsighted, the umpire decides; if no decision can be made, a let is played, unless instant review is available.
One rule underpins all of it: the line is in. A shuttle whose base touches any part of the line has landed in the court. Our in or out guide covers singles versus doubles lines and service boundaries.
Badminton Hawk-Eye and the instant review system
The BWF Council approved an experimental law for an Instant Review System (IRS) in April 2013, and it was first used at the BWF World Superseries Finals in Kuala Lumpur in December 2013, with each player or pair allowed two challenges per match. In April 2014 the BWF announced Hawk-Eye Innovations as its instant review provider, and Hawk-Eye made its badminton debut at the India Open in New Delhi that month.
Systems like Hawk-Eye use several synchronised high-speed cameras positioned around the court. Because each camera sees the shuttle from a different angle, the software can triangulate its position in 3D at each moment, reconstruct the flight path and estimate where the base touched the floor. That multi-view geometry is what a single camera cannot do.
The badminton challenge system: current rules
The rules have changed since 2013. Under BWF Statutes Section 4.1.8 (version 2.0, in force from 11 November 2023), on a court where the system operates:
- A player or pair may challenge a line judge’s call, or a call or overrule by the umpire.
- The challenge must be made immediately after the player sees the call.
- Challenges are unlimited until two fail in the same game. A successful challenge is kept; two unsuccessful challenges end the player’s or pair’s right to challenge for the rest of that game.
- The umpire can also request a review if a line judge was unsighted and the umpire can’t decide.
- If the system shows “No decision” or malfunctions, nobody loses a challenge; on “No decision” the original call stands (or a let is played if the original call was “unsighted”).
Instant review covers line calls; service faults are still judged by the service judge. Instant review only exists on courts where the equipment is installed, which at most events means the TV courts.
Try it: you be the line judge
Here is the problem every camera, and every line judge, faces. The shuttle touches the floor at an instant that falls between video frames, a little blurred, and then skids or tips over. Pick a frame rate, a resolution and a shot, drop a shuttle, and call it. Then see the truth and how wide the camera’s honest uncertainty band really is.
You be the line judge
You: 0/0 right
Close-up of a 60 × 30 cm patch around a 4 cm sideline (white). Faded shapes are earlier frames; only the frames the camera captured are shown.
Illustrative model, not a real tracker: horizontal speeds of roughly 3 m/s (falling clear) and 20 m/s (smash), shutter open for half of each frame, a few centimeters of skid after landing, and the stated centimeters per pixel. The uncertainty is half the gap between the last frame before and the first after contact, plus half a pixel and half the motion blur.
Play a few rounds at 30 fps with a smash, then at 240 fps with a falling clear. The first is a coin toss inside a band many centimeters wide; the second is often resolvable to about a centimeter. That is the whole story of camera line calling in one widget.
Why a single phone camera has limits
Resolution
A 1080p frame is 1,920 pixels wide. Stretched across a court seen from behind the baseline, a single pixel covers roughly a centimeter or more on the floor, and more in the far corners. A line is only 4 cm wide (BWF Laws), so a close call can come down to one or two pixels.
Frame rate
The moment of contact almost never lands exactly on a frame. A falling shuttle near the end of a clear drops at roughly 7 m/s, close to its terminal velocity according to aerodynamic studies, and still moves sideways. At 30 fps it moves centimeters between frames; after a smash it can move tens of centimeters. Software has to interpolate, and interpolation has error.
Motion blur
Each frame is an exposure over time, so a moving shuttle is a smear, not a point. Higher frame rates mean shorter exposures and sharper shuttles, but darker images under hall lighting.
Perspective
From one viewpoint, depth is compressed. Distances along the line of sight (for example on the far baseline) are squashed into a few pixels, and a shuttle in the air can look as if it is on the floor further away. Filming higher helps a lot; see the planner in how to film badminton.
Occlusion and the shuttle itself
Players’ legs block the landing spot, and a shuttle can land on its skirt, tip and slide. Even an official line judge sometimes has to signal unsighted.
Multi-camera review vs one phone
| Instant review (pro events) | One phone behind the baseline | |
|---|---|---|
| Cameras | Several, synchronised, around the court | One |
| Position in 3D | Triangulated from multiple views | Inferred from court geometry |
| Frame rate | High-speed cameras | Whatever the phone records (30–240 fps) |
| Calibration | Installed and checked for the event | Court found automatically or marked by you |
| Close calls | Resolved, with “No decision” possible | Too close to call within its uncertainty |
| Where | TV courts at major events | Any court, any club night |
The phone is not trying to be Hawk-Eye. It is trying to settle the obvious calls instantly, flag the genuinely close ones honestly, and give you a record of where every shuttle landed, which no line judge ever could.
Why “too close to call” is the honest answer
Every measurement has an error bar. If a system’s best estimate of the landing point is 1 cm outside the line, but its uncertainty is ±3 cm, it does not know whether the shuttle was in or out. Reporting “OUT” anyway is a guess dressed up as a measurement. Saying “too close to call” is not a failure; it is the system telling you the truth about what it can see.
Want to test your own rules knowledge on close calls, lets and faults? Try the badminton rules quiz or read faults and lets.
Frequently asked questions
Is the line in or out in badminton?
In. Under the BWF Laws of Badminton, a shuttle that lands on any part of a boundary line has landed in the court.
Does badminton use Hawk-Eye?
Yes. The BWF introduced an Instant Review System at the 2013 World Superseries Finals and contracted Hawk-Eye Innovations in 2014; Hawk-Eye debuted at the 2014 India Open. It is used on courts at major events where the equipment is installed.
How many challenges do you get in badminton?
Under BWF Statutes Section 4.1.8 (in force from 11 November 2023), a player or pair can challenge as often as they like until two challenges have failed in the same game. Successful challenges are not lost. When the system was introduced in 2013, the limit was two challenges per match.
Can the umpire overrule a line judge in badminton?
Yes. Law 17.5.1 says the umpire shall overrule a line judge if it is beyond reasonable doubt that the line judge has clearly made a wrong call. Where instant review is in operation, it decides any line call challenge.
What happens if the line judge can't see where the shuttle landed?
The line judge signals unsighted and the umpire makes the decision. If the umpire can't decide either, a let is called, unless instant review is available, in which case the umpire can request a review.
Can a phone camera make accurate line calls?
For clear-cut landings, yes. For landings within a few centimeters of a line, one camera is limited by resolution, frame rate, motion blur and perspective. An honest app reports those as too close to call rather than guessing.