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I have connected my Acer Chromebook to external screen and keyboard. How can I control the display brightness? (the screen does not have its own controls.) Usually I can click the brightness buttons in the built in keyboard but here I am using an external keyboard and also setting the built-in keyboard brightness only affects the built in screen and not the external one.

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From man xrandr:

      --brightness brightness
          Multiply  the gamma values on the crtc currently attached to the
          output to specified floating value. Useful for overly bright  or
          overly  dim outputs. 

You would use it like this:

xrandr --output <output> --brightness <brightness>

First, you need to find the name of the output:

xrandr -q

There should be several results displayed, with at least two of those marked "connected". My external VGA screen is called "DP1".

For brightness values, "1" is default; "2" is extra bright, "0.5" is dim-ish.

So for me, the command for a little extra brightness looks like this:

xrandr --output DP1 --brightness 1.2
  • how to run this command? do I need to be in developer mode? corsh does not recognize it – Yaron Naveh Jul 1 '15 at 8:57
  • Yes, you would need to enter developer mode, then enter the command shell, which should give you a full bash shell (and hopefully xrandr) – 3n4rch3 Jul 3 '15 at 7:19
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    It has been rather a while since ChromeOS offered xrandr. It uses wayland now. There is no known solution to the brightness problem. – JellicleCat Jan 6 '18 at 4:30

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