Fix brightness controls in Ubuntu 14

In some instances of Ubuntu 14, brightness control keys do not work. To correct this, first determine the type  of video card adapter controlling the display backlight; use:

$ ls /sys/class/backlight
intel_backlight

sudo vim /usr/share/X11/xorg.conf.d/20-intel.conf

Add the following lines to this file:

Section "Device"

        Identifier  "card0"
        Driver      "intel"
        Option      "Backlight"  "intel_backlight"
        BusID       "PCI:0:2:0"
EndSection

Save it. Log out and log in back. The brightness control should be working through function keys now: