must be disregarded by the video ref, as such judgements cannot reliably be made due to camera angle effects. Rugby union Use of video referee by referees was introduced to rugby union in 2001. The laws of the game allow for "an official who uses technological devices" to be consulted by the referee in decisions relating to scoring a try or a kick at goal. The decision to call on the video referee (now called "Television Match Official (TMO)") is made by the referee following discussion with the assistant referees/touch judges and cannot be instigated by the players or coaches of either team. When concerning an act of foul play, the TMO may alert the referee and initiate the replay process. In a possible try/no try situation, the referee shall signal his initial on-field decision (the "soft signal") and request the TMO to review all available footage and provide "advice and recommendations" to the on field referee. The referee should only change their decision where there is "clear and obvious" evidence that it was incorrect. In stadia with screens, the TMO may show footage directly to the referee. As per Law 6.5 A of the Laws of Rugby Union, "The referee is the sole judge of fact and of law during a match". Once a final decision is made, it is to be signalled by the referee. Tennis In tennis, systems such as Hawk-Eye and MacCAM calculate the trajectory of the ball by processing the input of several video cameras. They can play a computer rendering of the path and determine whether the ball landed in or out. Players can appeal to have the system's calculation used to override a disputed call by the umpire. In March 2008, the International Tennis Federation, Association of Tennis Professionals, Women's Tennis Association and Grand Slam Committee agreed unified challenge rules: a player can make up to three unsuccessful challenges per set, and a fourth in a tie-break. Television broadcasts may use the footage to replay points even when not challenged by a player. See also Photo finish Multicam (LSM), remote controller used for instant replays with XT3 servers. References Telecommunications-related introductions in 1955 Canadian inventions Sports television technology Sports officiating technology Rules of basketball National Hockey League on television Major League Baseball on television College football on television Slow motion Film and video technology