"acpilight" is a backward-compatibile replacement for xbacklight, that uses the ACPI interface to set the display brightness.

On some modern laptops "XRandR" might lack the ability to set the display brightness. This capability was moved/unified to the kernel's ACPI interface, via /sys/class/backlight/.

"xbacklight" provides a drop-in replacement for the same command, using the ACPI interface instead of "XRandR" in order to let old scripts run. As a result, "xbacklight" can now also be used from the console and Wayland (X11 is not used at all).

Normally, users are prohibited to alter files in the sys filesystem. It's advisable to setup an "udev" rule to allow users in the "video" group to set the display brightness.

To do so, place a file in /etc/udev/rules.d/90-backlight.rules containing:

SUBSYSTEM=="backlight", ACTION=="add", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
SUBSYSTEM=="backlight", ACTION=="add", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"

to setup the relevant permissions at boot time.