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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)?

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see this question on superuser: superuser.com/questions/256084/… –  Gerhard Burger Jan 8 '13 at 10:55
    
There is also this very neat software solution: askubuntu.com/a/48952/41499 –  Gerhard Burger Jan 8 '13 at 11:11

3 Answers 3

ADditional info: I'm using CRT monitor. – Neilvert Noval Mar 11 '11 at 17:06It seems that it is not possible to control external monitors using the /sys/class/backlight. But there exists an application called ddccontrol which can control the brightness of your external monitor.

Instructions on how to use the command line interface are available here.

Basically you need to probe for available monitors and controls using ddccontrol -p and look for the address of the control you want in the output and then set the value using ddccontrol -p -r 0x10 -w 70.

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Install Brightness Controller Install brightness-controller from the Ubuntu Software Center. If it is not available then download the .deb file.

Screenshot 1

Now you can adjust the brightness using the slider.

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This is not in the normal repos, but is in the software center at least for 12.04. It does not change the backlight brighness, but does change the overall brightness, though I'm not sure how. It says it uses a proprietary license, but also says it is "gpl-3". Very odd. And I'd like a command-line interface that I can script to gradually lower brightness in the evening. But I'm still glad to have it - thanks! –  nealmcb Apr 28 at 20:30
1  
It's written in Python and uses xrandr. We don't know how to package it for Ubuntu so we submitted it under propritary license (so that reviwers will package it). Source is available at github. github.com/lordamit/Brightness/tree/master –  Archisman Panigrahi Apr 29 at 7:07
1  
You can control brightness with xrandr via command line like askubuntu.com/a/457281/124466. That's what Brightness Controller does in the backend. –  Archisman Panigrahi Apr 29 at 7:13

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.

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. See

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