According to Alain Hamer a referee should be invisible during a match. Only when you haven't noticed him, his job has been well done.
Invisibility, though, is not always possible for the Luxemburger, considering his tendency to sometimes show an avalanche of yellow cards. For instance 10 in the 2005 CL mach Real Betis - Chelsea FC (1-0) and 8 in 2006 in the CL match Villarreal CF - Glasgow Rangers (1-1).
Invisibility? Take a look at this list:
2006 Villareal - Rangers: 53 fouls
2005 World Cup u17 Brazil - Korea: 66 fouls (yes, extra time, but still...)
2004 Champions League Olympiakos - Real Sociedad : 57 fouls
2002 Champions League Leverkusen - Fenerbahce : 57 fouls
Usually, in a non-Hamer match, the amount of fouls is between 20 and 35, sometimes 40.