How do I turn off the backlight but leave the LCD on?

Question

How do I set zero brightness on laptops that have a nonzero minimum brightness on the brightness control?

The solution should work regardless of X. I.e. it should also work in the virtual terminals.

Answer 1

From X Windows

From X Windows you can use the application xbacklight to get and set the percent brightness for your screen.

Example

current level

$ xbacklight -get
100.000000

set to 75%

$ xbacklight -set 75
$ xbacklight -get
73.333333

set back to 100%

$ xbacklight -set 100
$ xbacklight -get
100.000000

From the console

To achieve something similar for a virtual terminal you'll likely need to interact with the ACPI settings via the /sys filesystem.

Example

dims it

$ echo "10" | sudo tee /sys/class/backlight/acpi_video0/brightness 
10

full brightness

$ echo "15" | sudo tee /sys/class/backlight/acpi_video0/brightness 
15

You'll have to play with this one, the range of brightnesses can be from 0-9 or 0-15, I believe it ultimately depends on your laptop.

None of this works?

If neither of these 2 options suites your needs take a look at the ArchLinux Wiki's topic titled: Backlight. That article contains every method I've ever seen employed to achieve this!

References

• xbacklight man page
• How can I change brightness through terminal

Answer 2

In the realm of laptops with Intel chipsets, there seems to be a division between the LVDS- and eDP-connected panels:

eDP:

echo 4 > /sys/class/backlight/intel_backlight/bl_power

…should turn off the backlight.

echo 0 > /sys/class/backlight/intel_backlight/bl_power

…should turn it on again.

LVDS:

I have discovered that it is not possible to turn off the backlight on my Thinkpad X201 and X201 tablets via sysfs (bl_power does nothing; apparently, it is eDP-specific).

This page has a very lengthy description of the convoluted history of the backlight control in the linux kernel. TL;DR:

i915 has supported bl_power for eDP panels (but not LVDS) starting from v3.18.

HOWEVER, I have been able to make the backlight turn off by using a tool called intel_backlight from intel-gpu-tools. This requires root, since it apparently writes to a raw register.

intel_backlight 0

...turned the backlight off for me. Bumping it up with brightness adjustment controls re-enabled it here.

I also wrote a script that I hooked up to fire on a keyboard-generated ACPI event (in my case, fn+space, or button/zoom. If your system doesn't use that ACPI event, you need to use acpi_listen to find one that your system does have.

To trigger it, I made a file called fnspace-backlight in /etc/acpi/events:

# Wyatt Ward
# hook for magnify acpi event (fn+space)
# toggle LCD backlight on/off
event=button/zoom
action=/etc/acpi/actions/toggle-lcd-light.sh

I also made a file called /etc/acpi/actions/toggle-lcd-light.sh, marked as executable. This lets me toggle the backlight without changing the previously set brightness level, storing the temporary value in /brightness. Since ACPI events run actions as root, be careful.

#! /bin/bash
BRIGHTSAVEFILE="/brightness"
BRIGHTSYSFS="/sys/class/backlight/acpi_video0/brightness"

# is the light on or off?
INTEL_BACKLIGHT="/usr/bin/intel_backlight"
light_state=$("$INTEL_BACKLIGHT" | sed 's/current backlight value: //g'|sed 's/%//g')
echo "light: ""$light_state"
if [ "$light_state" -eq 0 ]; then
  cat "$BRIGHTSAVEFILE" > "$BRIGHTSYSFS"
else
  # back up current brightness level
  cat "$BRIGHTSYSFS" > "$BRIGHTSAVEFILE"
  # turn off backlight
  "$INTEL_BACKLIGHT" 0
fi

Ad an additional fun note, you can control backlight with an incredible amount of granularity by modifying intel_backlight slightly. Changing it to read arguments as floating point numbers rather than integers, and to do floating point arithmetic, only requires tweaking a few lines of code and lets you make ridiculously precise brightness changes, including ones lower than the minimum you can get via sysfs.