When Pierluigi Collina leads the teams out for tonight’s semi-final, it will be the last big occasion for the most famous football referee of the past two decades — and the most recognisable ever.

With his completely bald head, bulging eyes and cadaverous face, he could be every footballer’s nightmare. But Collina, who turns 45 (the retirement age for international referees) next February, has become the footballer’s best friend by sheer dint of his professionalism.

It’s a contradiction in terms: a good referee is supposed to be one who’s not noticed, whose whistle is not heard, who allows the game to flow without needless interruption. Collina is all of the above, which he accompanies with an easy grin, a pat on the back — and a dictatorial mien.

In an age where authority in all forms of sport are questioned, the football referee’s is surely the worst lot. Especially the top international referees, who officiate over players earning 20 times their salary. How they win the players’ respect, and retain it in the face of controversial, or simply unpalatable, decisions, is to the referees’ credit.

If television has made stars out of referees like Collina, and his alter ego the bleached-blonde pinup boy Anders Frisk, it can make villains out of those who give ‘‘bad’’ decisions. Ask Urs Meier, whose inbox was flooded with mail after the England-Portugal match, when he disallowed a ‘‘goal’’ by Sol Campbell. To hold your head high, walk with dignity, is not easy. Even Steve Bucknor is spared that ignominy.

Collina, though, doesn’t have those problems. His authority hasn’t come easy; it’s the product of careful and laborious homework. British referee Graham Poll once recalled preparing for a match during World Cup 2002 (he was the fourth official). Collina took Poll and the linesmen through both teams, drawing tactics and gameplans on the board, discussing styles of play, pointing out possible flashpoints and advising each official on where they should be when. It was almost as if a coach was briefing his team on their rivals.

‘‘It’s about tactics’’, he said on the UEFA website. ‘‘It’s completely different refereeing a match where a team plays with three players at the back or four, with tough midfielders pressing their opponents or wingers who play out wide.’’

A completely different approach to Frisk, who referees the first Euro 2004 semi-final. The Swede does no homework, preferring to rely on instinct and react to on-field happenings.

What they both share, as do all top referees and their assistants, is a fetish for fitness. They consider themselves athletes, too, and so expect a certain level of treatment. Collina is known to bust a gut if a referees’ room is dirty or below standards.

So is tonight the last we see of him? Not really; he’ll still be up for Champions League matches till February. And there’s a plan to bring him to England, to referee in the Premiership, where the retirement age is 48.