Sometimes, ACPI does not work well due to different motherboard implementations and ACPI quirks. This includes some laptops with dual graphics (e.g. Nvidia/Radeon dedicated GPU with Intel/AMD integrated GPU) – some examples of laptops with these problems are Dell Studio, Dell XPS 14/15/17 and some Lenovo series’ laptops. Kamal Mostafa, a kernel developer made patches for solving this problem – these are included after Linux 3.1 . Additionally, on Nvidia-optimus laptops, the kernel parameter nomodeset can interfere with the ability to adjust the backlight. You can try adding the following kernel parameters in your bootloader(grub, syslinux…) to adjust ACPI model:
Note: This kernel setting was added in Linux 3.13.