The Decision Review System is a technology-based system used in the sport of cricket. The system was first introduced in Test cricket, for the sole purpose of reviewing controversial decisions made by the on-field umpires in the case of whether or not a batsman had been dismissed. The system was first tested in an India v Sri Lanka game in 2008. The system was officially launched by the International Cricket Council on 24 November 2009 during the first Test match between New Zealand and Pakistan at the University Oval in Dunedin. It was first used in One Day Internationals in January 2011, during England's tour of Australia. The ICC initially made the UDRS mandatory in all international matches, but later made its use optional, whereby the system would only be used if both teams agree. The ICC has agreed to continue to work on the technology and will try to incorporate its use into all ICC events.
There have been basically three components in UDRS. The use of Snickometer was suspended but was reintroduced in 2013 while Hot spot is not used anymore.
Hawk-Eye, Eagle Eye, or Virtual Eye: ball-tracking technology that plots the trajectory of a bowling delivery that has been interrupted by the batsman, often by the pad, and can determine whether it would have hit the wicket or not.
Hot Spot: Infra-red imaging system that illuminates where the ball has been in contact with bat or pad.
Real time Snickometer, which relies on directional microphones to detect small sounds made as the ball hits the bat or pad.
How does the DRS work?
Each team is allowed to make no more than two unsuccessful review requests per innings during a Test Match and no more than one unsuccessful review request per innings during a One Day International. A fielding team may use the system to dispute a "not out" call and a batting team may use it to dispute an "out" call. The fielding team captain or the batsman being dismissed invokes the challenge by signalling a "T" with the arms. Once the challenge is invoked, acknowledged, and agreed, the Third Umpire reviews the play. At their discretion, field umpires may request the Third Umpire for certain close calls such as line calls (to determine run outs and stumpings), boundary calls (to see if a batsman hit a four or a six), or for close catch calls where neither umpire is sure if a catch was made. A challenge is always used in situations that did or may result in a dismissal: for example, to determine if the ball is a legal catch (making contact with the batsman's bat or glove and not touching the ground before being held by a fielder) or if a delivery made the criteria for a leg before wicket dismissal (hitting the ground in line or on the off side and hitting the batsman in line with a path that would have hit the wicket). The Third Umpire then reports to the on-field umpire whether his analysis supports the original call, contradicts the call, or is inconclusive. The on-field umpire then makes the final decision: either re-signalling a call that is standing or revoking a call that is being reversed and then making the corrected signal. Each team can initiate referrals up to the limit on unsuccessful reviews.
Under the DRS rule only clearly incorrect decisions are reversed; if the Third Umpire's analysis is within established margins of error or is otherwise inconclusive, the on-field umpire's original call stands.
When a not-out LBW decision is evaluated, and if the replay demonstrates the ball has made impact more than 2.5 m away from the wickets, various additional criteria apply to account for the uncertainty of the ball's potential direction after pitching. For example, if the ball pitches more than 2.5 m from the wicket and travels less than 40 cm before hitting the batsman, then any not-out decision given by the on-field umpire stands. It has also been decided that if the batsman is more than 3.5 m from the wicket, then not-out decisions will stand. The only picture in which an LBW decision will be reversed in favour of the bowler is if the batsman is 2.5–3.5 m away from the wicket and the ball travels more than 40 cm after pitching before hitting the batsman. In that case, some part of the ball must be hitting the middle stump, and the whole ball must be hitting the stumps below the bails; otherwise, the result is again inconclusive and the call stands. In cases where the original decision is out, the 2.5 m or 40 cm distances do not apply, as in that state Hawk Eye must show the ball to be completely missing the stumps in order for the umpire to undo his decision.
In 2013, ICC tested a broadcaster-free replay system. Under the experiment, a non-match umpire sits in a separate room with a giant monitor and has discretion over which replays to see rather than relying on the broadcaster. The non-match umpire mirrors the role of the third umpire without having the duty of making adjudications. The system was first used in an Ashes test (where Nigel Llong performed the duties of non-match umpire) and was repeated in a Pakistan-Sri Lanka ODI.