Brightness Controller is a simple app that lets you adjust external monitor brightness independently of your main display.
Better yet, the software dimmer also gives you a much greater control in how much you adjust the brightness.
For example, your laptop’s monitor brightness keys probably punch up or down in specific increments, e.g. 100%, 80%, 60%, etc, but Brightness Controller lets you change brightness to anything you like, from 1% to 100%!
The app also lets you set the color temperature of display and tweak the RGB values per-display.
This all works via
xrandr, so if you’re using a Wayland session this app, alas, won’t work.
Install Brightness Controller on Ubuntu
Brightness Controller is free, open-source software. It available to install on Ubuntu (and related distros) from a community PPA (which supports Ubuntu 18.04 LTS and above):
sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt update && sudo apt install brightness-controller
I don’t have a multi-monitor set-up of my own so I can’t attest to how well the app functions. But on paper, from everything I’ve read, it sounds like it should work and work well (plus it saves you needing to prod your monitor trying to find hardware keys).
If you try this app do feel free to share your experiences, thoughts in the comments. Be sure to report bugs on the project’s Github issues page.