Actually there are many ways to adjust the backlight of laptop screen. Sometimes only some of them are available on some kinds of the laptop. This article tries to summarize all possible ways to adjust the backlight.
It is possible to set the register of the graphic card to adjust the backlight
setpci -s 00:02.0 F4.B=0
It is possible to adjust the laptop by ACPI. But sometimes ACPI doesn't work well due to different implementation on mother board. You could use following command in grub to adjust ACPI model.
You also could adjust the backlight through the xorg-server command.