Pre Ubuntu LTS 16.04
Under (X)Ubuntu 14.04 LTS, brightness control on my Panasonic Toughbook CF-52 used to work fine by adding
acpi_osi=Linux to the GRUB_CMDLINE_LINUX_DEFAULT= line in /etc/default/grub:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"
and issuing
$ sudo update-grub
after editing and before rebooting.
Since Ubuntu LTS 16.04
However, above solution stopped working after upgrading to (X)Ubuntu LTS 16.04.
This appears to be typical for laptops with integrated Intel video graphics.
To find out if you have integrated Intel video graphics, enter the following command:
$ ls /sys/class/backlight/
intel_backlight panasonic
At least intel_backlight should be mentioned, most probably in addition to an OEM name like for example panasonic, dell_backlight, etc.
If this is the case, proceed with creating the following file
$ sudo nano /usr/share/X11/xorg.conf.d/20-intel.conf
containing the following lines:
Section "Device"
Identifier "card0"
Driver "intel"
Option "Backlight" "intel_backlight"
BusID "PCI:0:2:0"
EndSection
Reboot, and enjoy your backlight buttons!
Source