You can adjust the amount of minimum allowable light in the entire level by using the -ambient parm in the $light.exe compile command. For almost dark areas, I set mine to -ambient .0047 .0047 .0047 which barely lights up the dark areas of the map. I'm guessing that a setting of -ambient 1.0 1.0 1.0 would give the fullbright effect you are looking for. The settings are very sensitive though, as you'll discover if you try it. I won't ask why you would want to do this.