Do you mean Night vision so where you look or just ar adius around you?
I mean like a green playerlight, that only you can see and have a bigger radius than the normal playerlight.
It's the easiest way to do it, i think.
EDIT: I would not like it so much if it where completely client-sided, because then you could cheat on dark dm's, zombie survivals and such.
I would like it as an item, because then it can be placed to be used when you want it, and removed when you doesn't want it ect.