Find video/graphics card in Ubuntu and Linux Mint
Run the command below in terminal to know what video card is used for the backlight/brightness:
find graphics driver in Ubuntu
As you can see, the output for me is dell_backlight and intel_backlight. An indicator that the graphics card in use is Intel. Another way to find out the graphics card would be to go in System Settings->Details->Graphics. You can see the graphic card in use.
If your graphics card is Intel, you can proceed with the fix below.
Fix brightness control issue with Intel card in Ubuntu and Linux Mint:
Open a terminal and create the following configuration file, if it does not exist:
sudo touch /usr/share/X11/xorg.conf.d/20-intel.conf
Now we need to edit this file. You can use any editor be it a terminal one or graphical.
sudo gedit /usr/share/X11/xorg.conf.d/20-intel.conf
Add the following lines to this file:
Option "Backlight" "intel_backlight"
Save it. Log out and log in back. The brightness control should be working through function keys now:
Fix brightness control not working in Ubuntu 13.10