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I can switch the screen illumination manually off by pressing Fn+F6 on my laptop keyboard. This is the same as automatically switching it off on idle as it can be set in the power management.

The screen illumination will turn on again if I press any key. I want to keep on typing with a dark screen, i.e. I want to be able to switch it on again only by pressing one specific key combination. (In fact, external illumination e.g. from the sun behind my back can be sufficient to make the screen readable. This is also more eye friendly to me.)

How can I find which software mechanism turns the illumination on again on any key pressed so that I can restrict this mechanism to a particular combination?

This question seems not to be related to screen brighness as it is treated in this question. I can already adjust the brightness to be quite dark with another keyboard combination, but in the darkest setting /sys/class/backlight/acpi_video0/actual_brightness contains already 0 whereas the screen illumination is still brighter than off. So the switch I think about might be adressed by the Fn+F6 mechanism somewhere else than in the /sys/class/backlight/acpi_video0 tree. The files therein do not change when the screen illumination is turned off completely, only when the brightness is changed but still on.

I'm using MATE/Gnome on an old acer laptop, but hints for other system configurations might be helpful as well.

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Reset to default
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It depends on your video card and drivers. If the graphical brightness tools in Mate, gnome, kde and your keyboard keys do not work you have to manually edit the /sys/class/backlight file.

The location varies by graphics card , look in /sys/class as in the question you linked.

Some cards use values of 0-255 , others 0-10

You can script this if you wish

How to change LCD brightness from command line (or via script)?

Sometimes you can fix it by adding 

GRUB_CMDLINE_LINUX="acpi_osi=Linux acpi_backlight=vendor"

Update your question with better hardware information and I can update my answer.

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  • From the hardware side it all works. I just want to stop the software to catch every pressed key to switch the illumination on again when it is off.
    – Horst Grünbusch
    Nov 11, 2015 at 14:53
  • On all my laptops writing a value to blacklight does exactly what you ask. I doubt you want "actual_brightness", just "brightness" usually. Although they may be links
    – Panther
    Nov 11, 2015 at 15:50
  • They seem to be copies. They both change when I change the brightness. However, when I do sleep 3; cp brightness ~, press Fn+F6, then after 3 seconds diff -s brightness ~/brightness both files are the same, so brightness doesn't change if I switch the screen illumination completely off.
    – Horst Grünbusch
    Nov 11, 2015 at 20:29
  • @HorstGrünbusch - Identify your hardware please, update your question, and what driver are you using ?
    – Panther
    Nov 11, 2015 at 20:34

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