DRS Technology Explained: Hawk-Eye, UltraEdge and Ball-Tracking in Detail
How the Decision Review System (DRS) technology works in detail — Hawk-Eye ball-tracking (3D trajectory prediction with 5cm error margin), UltraEdge/Snicko (microphone and vibration detection for edges), Hot Spot (infrared friction detection), and the 'umpire's call' zone. What each technology can and cannot reliably detect.
The Decision Review System (DRS) uses multiple technologies to assist on-field umpires in making LBW, caught-behind, and caught-in-the-field decisions. Each technology has specific capabilities, limitations, and error margins. Understanding what the technology can and cannot reliably do — and what 'Umpire's Call' means in the specific context of each decision — is essential for following DRS reviews accurately. DRS was introduced into Test cricket in 2009 and is now mandatory at all ICC events; some bilateral series still do not use DRS due to cost or host board decisions.
Hawk-Eye: Ball-Tracking for LBW
Hawk-Eye is a camera-based ball-tracking system (developed by Hawk-Eye Innovations, now owned by Sony) that reconstructs the ball's 3D trajectory from multiple camera angles and predicts where it would have gone if not interrupted by the batsman's pad. For LBW decisions, DRS uses Hawk-Eye to assess: (1) Impact — did the ball hit the pad in line with the stumps? (2) Pitching — did the ball pitch outside leg stump (if so, not out); (3) Trajectory — would the ball have gone on to hit the stumps? Hawk-Eye's stated error margin is ±2.5cm in the x-axis (lateral) and ±5cm in the y-axis (height) for the trajectory prediction. The 'Umpire's Call' zone (the grey zone around the stumps where the ball is on the edge of missing) is set at approximately the width of the stumps in the lateral direction — if the predicted impact falls within this zone, the original umpire's decision stands.
UltraEdge and Hot Spot: Detecting Edges
UltraEdge (also called Snicko or Snickometer): Uses a highly sensitive directional microphone positioned at the pitch to detect sound waves at the moment the ball passes the bat. The audio waveform is displayed graphically — a spike at the moment of ball/bat proximity indicates contact. Limitations: the microphone cannot distinguish between the ball touching the bat vs the bat grazing the pad vs a pad/ground contact. Hot Spot: Uses two infrared cameras positioned behind each end of the pitch. When the ball rubs against any surface (bat, pad, glove, ground), friction generates heat that appears as a bright spot in the infrared image. Hot Spot can confirm edge contact if a friction mark appears on the bat edge at the correct moment. Limitation: very fine edges (which generate minimal friction) sometimes do not produce a visible Hot Spot mark — producing 'no mark' Hot Spot results that leave doubt.
What DRS Cannot Reliably Detect
DRS is not infallible across all decision types: (1) Bat-pad catches — where the ball hits the pad then the bat (or vice versa): UltraEdge may detect the contact but cannot determine the sequence. (2) Fine glancing edges on very fast deliveries: Hot Spot may not register sufficient friction; UltraEdge may not produce a clear spike. (3) Very thin deflections off a glove (particularly the bottom hand): the friction point is often at the glove-handle interface, not detectable by either camera. (4) Caught close catches (bump ball vs off the bat): Hawk-Eye only tracks the ball in the air; it cannot determine whether the ball bounced before reaching the fielder. For these edge cases, the umpire's original decision — supported or unsupported by technology — remains the operative decision. Teams are allowed 2 failed DRS reviews per innings; successful reviews do not use one of the two reviews.
Frequently asked questions
What does 'Umpire's Call' mean in DRS?
Umpire's Call applies when Hawk-Eye's ball-tracking prediction shows the ball hitting the stumps within the defined error margin — the grey zone approximately equal to the width of the stumps in the lateral dimension. If the predicted ball trajectory is 'Umpire's Call' in impact or trajectory, the original on-field umpire's decision stands (even if the ball appears to be hitting the stumps on screen). The rationale: Hawk-Eye's error margin is not zero, and within the error zone, the original umpire's decision should be respected. Teams retain their DRS review if the decision is 'Umpire's Call' (only reviews overturned by DRS consume a review).
How does Hawk-Eye track the ball's trajectory?
Hawk-Eye uses 6-8 cameras positioned at multiple angles around the ground to track the ball's 3D position at each frame (60-200 frames per second depending on camera speed). It uses the pre-impact trajectory (observed directly) to calculate the post-impact trajectory (predicted through physics modelling based on ball speed, angle, and spin rate). The prediction becomes increasingly uncertain with distance — the further beyond the impact point, the wider the error margin. Hawk-Eye's error margin is ±2.5cm lateral, ±5cm vertical at stump height.
How many DRS reviews does each team get?
In Tests: 2 unsuccessful reviews per innings. In ODIs: 2 unsuccessful reviews per innings. In T20Is: 1 unsuccessful review per innings. A review that succeeds (overturning the umpire's decision) does NOT count as a used review. A review resulting in 'Umpire's Call' also does NOT consume a review (the decision stands but no review is lost). Teams can use DRS for LBW decisions, caught-behind decisions, and caught-at-the-field decisions — they cannot use DRS for no-balls, wides, or run-outs (which have their own TV umpire referral process).