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Drew Noakes
Asked: 2013-04-17 15:11:59 +0800 CST

Setting a display's backlight brightness to be lower than is possible via the normal brightness controls

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By normal brightness controls I mean the accelerators on my laptop's keyboard that seem to integrate with Unity.

In a dark room, my screen is quite bright, even on the lowest setting. Can I override this?

I tried setting it explicitly via both:

sudo echo 500 > /sys/class/backlight/acpi_video0/actual_brightness
sudo echo 500 > /sys/class/backlight/intel_backlight/actual_brightness

But I get a permission denied error.

When at the lower bounds via my keyboard's brighness controls, acpi_video0/actual_brightness is 0, and intel_backlight/actual_brightness is 729. Can I set the latter value to be even lower?

display

3 Answers

  1. Best Answer

    Your command gives elevated permissions to the echo command (which doesn't need them), but not to the part that redirects the output stream to the root-owned file (which does)

    In a terminal, you can either do

    echo 7 | sudo tee /sys/class/backlight/acpi_video0/brightness

    which gives the elevated permission to the bit that really needs it, or

    sudo sh -c "echo 7 > /sys/class/backlight/acpi_video0/brightness"

    which runs the whole command in a sudo shell

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  2. I created a slider tool based on the information presented here in the question/answers:

    It provides an alternative slider to control backlight with finer control.

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  3. Turns out that the two items I show in the question are read only for all users.

    However /sys/class/backlight/intel_backlight/brightness is not. So, the following works:

    sudo su
echo 500 > /sys/class/backlight/intel_backlight/brightness

    Be aware that echoing a zero value makes the screen is unreadable :)

    You may have a different name for your device (instead of intel_backlight).

    Also, performing the above using sudo echo ... doesn't work. Can someone explain why I have to do sudo su first?

    EDIT @Vivek's answer explains this.

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