Changing the screen brightness of the external screen

It's very easy to do via the command line. First, type the following command in terminal to identify your screens:

xrandr -q | grep " connected"

and you'll get something like this:

LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 194mm

VGA1 connected primary 1366x768+1366+48 (normal left inverted right x axis y axis) 413mm x 234mm

LVDS1 is related to laptop's built-in display. I am using an external monitor (VGA1 in this case). If you want to reduce the brightness of external screen, you just type this, for example:

xrandr --output VGA1 --brightness 0.63

Hardware control solution (no software dimming)

By now there are 2 softwares to do hardware dimming:

• ddccontrol (CLI and an GUI)
• ddcutil (CLI and an GUI)
• ddcci-backlight (driver to be picked up by GNOME and others)

Tool 1: ddccontrol

ddccontrol (note the double cc) is a tool to control the settings of many monitors in exactly the same way their on-screen display / hardware buttons control them.

It is available in Ubuntu (man page) via apt install ddccontrol.

gddccontrol is a graphical user interface for it: apt install gddccontrol

Both need to be run as root:

• sudo ddccontrol for the command line tool
• gksudo gddccontrol or pkexec gddccontrol for the GUI tool.

(Based on @Ad Infinitum's comment in @Taz8du29's comment (but note and extra c in the name.)

Tool 2: ddcutil / ddcui

An alternative to ddccontrol, made at a time when ddccontrol was rather unmaintained.

It is available in Ubuntu (man page) via apt install ddcutil.

You can run them as root or install the i2c-tools and add your user to the group i2c to do it without root (explanation).

It also has a GUI called ddcui (screenshot here).

Tool 3: ddcci-backlight driver

This ddcci driver integrates all ddcci-capable monitors into sysfs, including /sys/class/backlight/. Because i.e. GNOME will use that interface to set the brightness, you can set the brightness without an additional UI, or the terminal.

It is available on Ubuntu: apt install ddcci-dkms

The brightness controller mentioned before is now version 2. The original simple version is available using the following steps with support for up to 4 monitors. Tested working without issue on Ubuntu 14.04

sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt-get update
sudo apt-get install brightness-controller-simple

Enjoy!