All I had to offer (which I have only heard, and can't find anywhere) was that a hockey goalie helmet couldn't be used. My understanding for this is that the back of a hockey goalie helmet doesn't provide enough protection as the chances of being hit in the back of the head are slim as they don't play behind the goal.
The FIL rules state:
Rule 17 Personal Equipment
17.1 All players are required to wear protective gloves, suitable boots or shoes, and a protective helmet equipped with a facemask and a chinstrap, which must be properly fastened on both sides.
... but that is all I can find.
Does anybody know if there are any more specific rules than that? Does the FIL approve certain helmet models or manufacturers (Brine/Warrior/Cascade etc.) or is it down to the referees interpretation? Under the above rule presumably and ice hockey or canoe polo helmet would be permitted, so long as the ball wouldn't fit between the face mask? What about a cricket helmet, I wouldn't play in one but it fits the above rule?