It doesn't sound like a fun experience at all mate. I have a peephole and outside light so seeing them wouldn’t be an issue. I have a few stout walking poles in the house that I would pick up in preference to a knife; if it truly came down to it I can do just as much harm with one, keeps the other guy a bit further off and feels much less premeditated.
I'd certainly use the security chain before opening the door, even if I felt there was little threat.
I also rather like Spork’s idea of the fire extinguisher; I have a little powder one in the kitchen that would certainly be a deterrent if sprayed in someone’s face.
Above all though, there is no point in taking risks when you have a family to think of.