Symptom: You installed the mod pak, or cave vision alone, but caves are still dark.

Fix:

1. First try the FBO fix.
2. If that doesn't work, log off, go into Settings from the launcher (the gear icon), and under Graphics set the renderer to Legacy. Log in, you should have full bright caves. Note, yes you don't have some of the advanced graphics features, but you can see in caves. It's a trade off.