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: