How do I open/edit/create .rad files?

Kazagin

L3: Member
Aug 4, 2009
130
9
(Topic)
I need to create a lights.rad file so that I can make a certain texture glow in my map.
Unfortunately, I have no idea how to use .rad files.

So yeah, what program do I need to open, edit, and create .rad files?
(I tried Google, but that only got me a bunch of crap that doesn't help me use .rad files.)