From https://wiki.archlinux.org/index.php/backlight#sysfs_modified_but_no_brightness_change
Works for Laptop msi GT72VR 6RD Dominator
Note: This behavior and their workarounds have been confirmed on the Dell M6700 with Nvidia K5000m (BIOS version prior to A10) and Clevo P750ZM (Eurocom P5 Pro Extreme) with Nvidia 980m.
On some systems, the brighness hotkeys on your keyboard correctly modify the values of the acpi interface in /sys/class/backlight/acpi_video0/actual_brightness but the brightness of the screen is not changed. Brigthness applets from desktop environments may also show changes to no effect.