How do I disable Automatic Brightness in Ubuntu-Gnome [16.04/15.10]?
I realize this is a bit late but I was googling for this solution and I found the setting when I was messing around. Settings > power then turn automatic brightness off ! Hope it helps.
In Ubuntu 16.04, if you go to All Settings > Brightness & Lock, there is an "Dim screen to save power" checkbox. Uncheck this option and the computer should stop attempting to change the brightness automatic. There is also an option on the same page right above this checkbox to manually set the brightness. Hope this helps!