Por esta resposta no Quora , tente isto:
Install xbacklight program by typing the following in the terminal:
sudo apt-get install xbacklight
Now press alt+f2 from your keyboard, and type gnome-session-properties. Click on the application, and from the window popped, click add, and fill the following:
name = Brightness Fixed
command= xbacklight -set 25
clique em adicionar e pronto. Reinicie para verificar os efeitos.