Having the same problem. GPU is Ivy Bridge HD4000. Installed the latest driver v. 3165. Quake 3 is way too dark. Tried changing gamma setting with no result.
I noticed that this only happens when using full screen. Searching on the internet brings many posts on OpenGL games. Finally found somewhere the solution. Changed this value in configuration file to
r_overbrightbits 0
after that brightness slider works again.
Should work for Sandy Bridge as well.
Good Luck