Backlight
Backlight control of liquid-crystal displays (LCDs) screen is covered in this page.There are multiple utilities available for this purpose.
brightnessctl is one popular package. Refer Brightness control for more information.
Ddcutil
The command ddcutil from the package ddcutil can be used to control brightness in external monitors, if your hardware supports it.
Once the package is installed, check the current brightness of your monitor by issuing the command
ddcutil -d 1 getvcp 10
To set brightness of your monitor to 50% issue the command:
ddcutil -d 1 setvcp 10 50
Troubleshooting
No /dev/i2c devices exist
The command ddcutil may generate the below error:
Unexpected error. Unable to open sysfs directory /sys/class/drm/card1-HDMI-A-1/ddc/i2c-dev: No such file or directory Unexpected error. Unable to open sysfs directory /sys/class/drm/card1-HDMI-A-2/ddc/i2c-dev: No such file or directory No /dev/i2c devices exist. ddcutil requires module i2c-dev.
If you receive the above error, on running ddcutil, then load the i2c-dev kernel module:
# modprobe i2c-dev
Rebooting the computer after installing ddcutil should automatically load the above module based on /usr/lib/modules-load.d/ddcutil.conf.