Open a terminal, enter the command:
$ sudo gedit /etc/default/grub
Find code like this:
$ GRUB_CMDLINE_LINUX=""
Modify this code to
GRUB_CMDLINE_LINUX="acpi_backlight=vendor"
Update GRUB
$ sudo update-grub
Set initial value of brightness
$ sudo gedit /etc/rc.local
Add this code before exit 0:
$ sudo echo 0 > /sys/class/backlight/acpi_video0/brightness
0-10, which corresponds to 0 to 100%, echo 5 means that half of the luminance. Save the file and exit, restart your computer.