Have you seen these cool ads for the ultra bright flashlights? They tout how you can use them as a defensive tool to "blind an attacker"?
This is a dangerous bunch of BS.
In a dark environment (like, your house at night), it's true that any bright light will disorient someone. The fact is, however, the light reflected off your walls will also affect your night vision. Is the attacker alone? How will you know? You just made sure that you can't see his partner in any other dark part of the house.
Also, what is keeping Attacker #1 from charging you? You just lit a beacon for him. Straight to you. Even if he's (temporarily) blinded, he can still strangle or stab you.
Here's a better idea, Use a standard (or low powered) light and flick it in the assailant's direction, for less than a second. You'll recover your night vision much quicker, and it will still scare the hec out of Thing #1. Thing #2 will have a harder time finding you. But they know you're there, somewhere. Not seeing you, they could start thinking you have just lowered the barrel of your shotgun on them. And, if you're bright, maybe you have. They will beat feet.
Option 2 involves buying a can of "bear spray". It shoots a cloud of pepper spray 20 feet. Flick the light to see where he is, blast the can, and listen to the gagging and screaming. An assailant can recover from a bright light in seconds. Tear gas takes a lot longer. And you can't do ANYTHING after being gassed. I've seen it.
Just use the right tool for the right job, and don't bet your life on short-cuts.