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The offside rule in soccer still defeats most mortals—including match
officials. Soccer referees can be fooled by parallax, as players dash into illegal positions
(New Scientist, 4 March, p 8). So Norwegian inventor
Kjell Hattland suggests in WO 99/47217 that assistant referees take more
responsibility for offside decisions. A line of light sources along one side of
each half of the pitch, are shielded so tightly they are only visible from a
narrow angle. This helps the official keep level with players, giving an instant
fix on the players’ position.