I've always pictured it like the Phosphorescent fungus in the movie version of Journey to the Center of the Earth. (Can't recall if it's in the book, but I think it is.) I also suspect this is where it was taken from. .
I don't recall any mention of this in the 2e books, but I used it all the time, and I got the idea from the movie
Basically the fungus gives off a soft glow equivilent to twilight, the glow fills the area where it's contained so there really is no radius, it's just how ever far the walls are coated you can see.
As for making it where humans can't see with this, what's even the point of having it. You use this so people don't have to carry torches, or at least I do. Made a whole city of underground humans once lit by this.