Reduce External Monitor Brightness on Ubuntu 20.04, 21.10

External Monitors are a great addition to any workspace. I have a Full HD Benq one connected with my laptop via HDMI cable to do multi-tasking.

What I don't like about this setup is the external screen is still very bright even after setting it to the lowest possible setting via physical buttons present on the monitor. Changing brightness on the Laptop doesn't affect the brightness of the external monitor.

However, In Ubuntu, you can adjust the brightness level of any connected monitor by opening up a terminal/command line and entering a few comments.

Xrandr is the command we would be using to achieve this.

How to change External Monitor Brightness on Ubuntu Linux

1. Open a Terminal ( Ctrl + Alt + T )
2. Enter the command xrandr -q | grep " connected" and press Enter. Find the external monitor name displayed in this command's output. In my case, it is HDMI-1
3. To change the brightness enter this command xrandr --output HDMI-1 --brightness 0.5 and press Enter. ( change the monitor name HDMI-1 with the one you have ). 

That's it. your brightness of your external monitor would be now reduced. You can change the 0.5 value to 1 if you want to increase the brightness or 0.2 if you want to reduce it even more.

Alternate Way using brightness-controller app

There is an app that lets you adjust the brightness of the external monitor. It's not available in the official Ubuntu repository, so you have to add it's PPA repository to your system first.

Enter the below commands

sudo add-apt-repository ppa:apandada1/brightness-controller

sudo apt-get update

sudo apt-get install brightness-controller

Once this Brightness Control app is successfully installed on your system, you can launch it from the app drawer. Press the Windows key on your lap and enter 'Brightness Controller' and launch the app from there. Adjust the settings to your taste. Using this app, you can also adjust RGB colors as well as Color Temperature.

Oct 15,2021 update: The Brightness Controller app works wells on Ubuntu 21.10 out of the box.

April 27, 2021 update: if you have updated your machine to Ubuntu 21.04, the below commands would no longer work as the new version uses Wayland by default. Check out how to reduce external monitor brightness on Ubuntu 21.04 from here.

I have tested this on Ubuntu 20.04 LTS Focal Fossa only. It should work on older 18.04 LTS, 18.10, 19.04, 19.10 as well as on future 20.10 versions as well.