sudo gedit /etc/default/grub
Edit the two lines (GRUB_CMDLINE_LINUX_DEFAULT and GRUB_CMDLINE_LINUX) to read as follows:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor vga=792"
GRUB_CMDLINE_LINUX="acpi_osi=Linux"
sudo update-grub
#need to reboot
#sudo reboot
echo 9>/sys/class/backlight/acpi_video0/brightness