How it arrived
The system was trialled in July 2008 during India's Test series in Sri Lanka, and the ICC adopted it formally in November 2009, first used in a Test between New Zealand and Pakistan in Dunedin. The Board of Control for Cricket in India declined to use it in its own series for several years, citing doubts about the ball-tracking projection, and agreed to a trial during England's tour of India in 2016. Its availability still depends on the host broadcaster's equipment, which is why a series in one country may have infrared cameras and a series elsewhere may not.
The components
- Ball tracking
- Multiple cameras reconstruct the ball's flight from release to the point of impact and project its path onward to the stumps. Hawk-Eye supplies it in many countries; Virtual Eye, from Animation Research in New Zealand, in others. The Hawk-Eye sheet covers the mechanism.
- Edge detection by sound
- Known as Snickometer or UltraEdge depending on the provider, this aligns the stump-microphone waveform with the video frames so that a spike can be matched to the moment the ball passes the bat.
- Hot Spot
- Infrared cameras from BBG Sports in Australia show the heat left by friction where ball met bat or pad. It is expensive and not present in every series.
- Replays
- Ordinary slow-motion footage for run-outs, stumpings, catches, boundaries and the front-foot no-ball.
Who may ask
A batter who has been given out, or the fielding captain when an appeal is turned down, may ask for a review by making a T shape with the arms within 15 seconds of the decision. In Tests each side has two unsuccessful reviews per innings; since 2017 these are not replenished after 80 overs, and a review that ends in umpire's call is not counted as lost. The allowance in one-day and Twenty20 internationals has been changed more than once, and the current ICC playing conditions are the place to check it rather than memory.
The umpires have their own route. On-field umpires can refer run-outs, stumpings, boundary decisions, whether a catch carried and, where the technology is in place, front-foot no-balls, which in international cricket are now checked by the third umpire on every delivery. Until 2023 the on-field umpire gave a "soft signal" of out or not out before referring a catch; the ICC removed that requirement in June 2023, so the third umpire now judges a referred catch on the footage alone.
The order of a review
- Legality of the delivery. The third umpire checks the front foot first. If it is a no-ball nothing else matters and the batter is not out.
- Bat involved? For an LBW review the third umpire checks the sound spike and, where available, Hot Spot, to see whether the ball touched the bat before the pad. An inside edge ends the LBW question.
- Pitching. The ball must pitch in line with the stumps or on the off side. A ball pitching outside leg cannot be out LBW whatever follows.
- Impact. Where the ball struck the batter must be in line with the stumps, unless the batter was not attempting a shot.
- Wickets. The projected path must go on to hit the stumps.
For impact and wickets the zone is defined by the outer edges of the off and leg stumps and the top of the bails, a definition widened in October 2016 from an earlier narrower zone. If more than half of the ball is inside the relevant zone, the graphic reports it as in line or hitting and the decision is settled by the technology. If less than half is inside, the graphic reports umpire's call and the on-field decision stands.
Working note
Umpire's call is the reason the same delivery can be out under one on-field decision and not out under the other. Critics call that inconsistent. Its defenders point out that the projected path is an estimate, and that a margin which defers to the umpire when the estimate is close is more honest than a hard line drawn through uncertainty. Both descriptions are accurate; they weight speed of understanding against fidelity to the evidence differently.
Why the margin exists
Everything before impact is measured; everything after it is projected. The shorter the measured flight between pitching and impact, the less data the model has, and the further the impact is from the stumps, the longer the projection has to travel. The ICC's playing conditions have over the years included specific cautions for those cases, treating projections with extra care when the ball travels only a short distance after pitching, or when impact is a long way, around two and a half metres, from the stumps. The wording has changed more than once, so quote the current text rather than an older summary.
Set against the other sheets, DRS is the protocol that has been most explicit about its own uncertainty. Tennis shows an in-or-out footprint with no margin; football's offside has no margin at all; rugby's TMO protects the on-field decision through the wording of the question. Cricket alone writes the tolerance down as a fraction of the ball and shows it on screen. Whether that makes it the best of the five or merely the most argued about is a fair question, and the answer depends on what you think a review is for.