Can I control brightness on second monitor?

I have a thinkpad (x301) with an external monitor connected via a DisplayPort to HDMI cable. I can control the brightness of the laptop monitor using the keys on the laptop, but I can't work out how to control the brightness of the external monitor.

$ ls /sys/class/backlight/*/brightness
/sys/class/backlight/acpi_video0/brightness
/sys/class/backlight/intel_backlight/brightness
$ cat /sys/class/backlight/acpi_video0/brightness
11
$ cat /sys/class/backlight/acpi_video0/max_brightness
15
$ cat /sys/class/backlight/intel_backlight/brightness
501804
$ cat /sys/class/backlight/intel_backlight/max_brightness
2414340

I can write values into the above and both intel_backlight and acpi_video0 changes the brightness of the laptop monitor.

Does the above mean that there is no way to change the brightness of the external monitor (other than the OSD)?

Solution 1:

You can do this from the command line (and thus do it automatically each evening via cron) using xrandr as described at How to change LCD brightness from command line (or via script)?

E.g.

xrandr --output HDMI1 --brightness 0.5

This doesn't change the backlight or affect power usage, it just adjusts the gamma value in software. I've heard that Brightness Controller uses xrandr under its GUI skin.

Use xrandr --listactivemonitors to get the correct monitor if not you'll probably get xrandr: Need crtc to set gamma on..

ddccontrol was in Lucid 10.04 and seems to have worked well, but was orphaned by Debian due to lack of interest and hasn't been picked up by anyone since.

Update ddccontrol is back in xenial. See

• #483479 - O: ddccontrol -- a program to control monitor parameters - Debian Bug report logs
• http://packages.ubuntu.com/search?keywords=ddccontrol
• Control external monitor brightness via software
• How to change external monitor brightness via DDC/IC?

Solution 2:

Install Brightness Controller from the following PPA.

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

Now you can adjust the brightness using the slider.