DRS Review System Fully Explained

The Decision Review System (DRS) is cricket's technology-aided umpiring review process — used in international cricket since 2009 to allow teams to challenge on-field umpiring decisions they believe are incorrect. DRS uses a combination of ball-tracking software (Hawk-Eye), edge detection technology (Snickometer and UltraEdge), and infrared heat-imaging (Hot Spot) to provide the third umpire with better information than is available to the on-field umpires with the naked eye. Each team is allocated a fixed number of reviews per innings, which they lose when a review is unsuccessful.

What Can Be Reviewed

Teams can review any on-field umpiring decision that results in a dismissal (or the absence of one): LBW decisions (for or against), caught-behind decisions, and any other caught decision. Teams cannot review no-balls (these are reviewed automatically by the third umpire in some competitions), and cannot review wide or no-ball decisions. The reviewing team signals their intent by drawing a T-shape in the air with their hands — the captain or the batter/fielder making the catch must signal the review within 15 seconds of the original decision. Once signalled, the third umpire reviews the available technology and communicates a decision to the on-field umpires.

Hawk-Eye Ball Tracking

Hawk-Eye uses multiple high-speed cameras positioned around the ground to triangulate the ball's trajectory from the moment of release to the moment of impact. For LBW reviews, Hawk-Eye models where the ball would have gone had the batter's pad not been in the way — whether it would have hit the stumps. The ICC applies 'umpire's call' for marginal LBW decisions: if the on-field umpire's decision falls within the margin of error of the ball-tracking prediction (the ball is predicted to hit partly in and partly out of the stumps, or pitch impact is close to the crease line), the on-field decision stands even after review. This umpire's call mechanism preserves the umpire's authority for genuine borderline decisions.

Snickometer and UltraEdge

Snickometer (and its modernised successor UltraEdge, used in most ICC events) uses a directional microphone positioned at the stumps and a synchronised oscilloscope waveform to detect whether the ball has made contact with the bat edge or another surface. When a batter is given out caught behind, a review can check whether an edge was actually present by examining the sound wave at the exact moment of ball-bat proximity. If no sound spike is present in the waveform at the moment the ball passes the bat, the on-field caught decision is overturned as not out. The sensitivity of modern edge-detection microphones is such that even very faint glances can be detected — though thick outer edges on bat handles and bat-pad collisions can cause confusion.

Hot Spot Infrared Technology

Hot Spot uses two infrared cameras positioned to cover both sides of the batter to detect heat generated by friction — when a ball brushes a bat or pad, friction generates a small heat spot that shows up on the infrared image as a bright area. Hot Spot can confirm an edge even when UltraEdge finds no sound evidence (sometimes a very thick edge creates no sharp sound but does generate heat). However, Hot Spot has limitations: it works best in cooler conditions (heat noise from the ball and bat can obscure marginal readings), and very fine nicks may not generate enough heat to register. Not all ICC events use Hot Spot due to the cost of the infrastructure.

Review Retention and Loss

Each team begins an innings with two DRS reviews. A review is lost if the third umpire finds the on-field decision was correct — meaning the challenging team was wrong. A review is retained if the on-field decision is overturned (the challenging team was correct) or if umpire's call is applied on any element of the decision (pitch or impact — in an LBW review where umpire's call applies, the challenging team keeps the review regardless of whether the decision changed). Teams often hold reviews in reserve for high-value decisions — particularly LBW decisions against their best batters — rather than spending them on marginal caught-behind reviews.