The interesting thing about tower shields, at least in 3.5, you receive total cover from all directions as there is no facing mechanic. This means by RAW spreads can't get past your tower shield as there are no corners to go around. Since you have total cover in a sphere around you even if you hold a lantern behind the shield it dysfunctionally doesn't shine anywhere except in your sphere of protection. This also works the other way too. You can't see beyond your shield while benefitting from total cover