Brightness should be controlled by pressing Fn + F5 and Fn + F6. Nothing happens when I press these keys. My computer is a Asus X551MAV-SX366GB and I'm using Lubuntu 14.04
Here is the summary of what I did:
I created the file
/usr/share/X11/xorg.conf.d/20-intel.confas described here 13.10 Can't Adjust Dell Laptop Backlight:
Section "Device" Identifier "card0" Driver "intel" Option "Backlight" "intel_backlight" BusID "PCI:0:2:0" EndSection
Then I changed one line in
/etc/default/grub so it looked like this: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=" and ransudo update-grub` from the command line.
Install xbacklight if not already installed
sudo apt-get install xbacklight. Then I put
xbacklight -set 50to autostart applications.
This worked for me.