One of the challenges for referees is that the human eye
can handle only approximately 16 images per second, which
means the ball needs to be behind the line for at least 60
milliseconds. However, in some cases the ball is only behind
the line for a few milliseconds before a player kicks it back
or it rebounds back into the fi eld of play, with the result
that the human eye cannot see whether the ball has crossed
the line. The ball can only be detected by the human eye at
a speed of 12km/h or less, whereas nowadays players are
able to shoot at a speed of over 120km/h (cf. “The hardest
recorded shot in football – ever”, 14 February 2007, The
Guardian)