Backlight
Screen brightness might be tricky to control. On some machines physical hardware switches are missing and software solutions may not work well. However, it is generally possible to find a functional method for a given hardware. This article aims to summarize all possible ways to adjust the backlight.
There are many ways to control brightness of a monitor, laptop or integrated panel (such as the iMac). According to these discussions and this wiki page the control method can be divided into these categories:
- brightness is controlled by vendor-specified hotkey and there is no interface for the OS to adjust the brightness.
- brightness is controlled by either the ACPI, graphic or platform driver. In this case, backlight control is exposed to the user through
/sys/class/backlightwhich can be used by user-space backlight utilities. - brightness is controlled by writing into a graphic card register through setpci.
Hardware interfaces
ACPI
The brightness of the screen backlight is adjusted by setting the power level of the backlight LEDs or cathodes. The power level can often be controlled using the ACPI kernel module for video. An interface to this module is provided via a sysfs(5) directory at /sys/class/backlight/.
The name of the directory depends on the graphics card model.
$ ls /sys/class/backlight/
acpi_video0
In this case, the backlight is managed by an ATI graphics card. In the case of an Intel card, the directory is called intel_backlight. In the following examples, acpi_video0 is used. If you use an Intel card, simply replace acpi_video0 with intel_backlight in the examples.
The directory contains the following files and subdirectories:
$ ls /sys/class/backlight/acpi_video0/
actual_brightness brightness max_brightness subsystem/ uevent bl_power device/ power/ type
The maximum brightness can be displayed by reading from max_brightness, which is often 15.
$ cat /sys/class/backlight/acpi_video0/max_brightness
15
The brightness can be set by writing a number to brightness. Attempting to set a brightness greater than the maximum results in an error.
# echo 5 > /sys/class/backlight/acpi_video0/brightness
By default, only root can change the brightness by this method. To allow users in the video group to change the brightness, a udev rule such as the following can be used:
/etc/udev/rules.d/backlight.rules
ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="acpi_video0", GROUP="video", MODE="0664"
For intel_backlight, use the following udev rule:
/etc/udev/rules.d/backlight.rules
RUN+="/bin/chgrp video /sys/class/backlight/intel_backlight/brightness" RUN+="/bin/chmod g+w /sys/class/backlight/intel_backlight/brightness"
Kernel command-line options
Sometimes ACPI does not work well due to different motherboard implementations and ACPI quirks. This results in, for instance, inaccurate brightness notifications. This includes some laptops with dual graphics (e.g., Nvidia/Radeon dedicated GPU with Intel/AMD integrated GPU). Additionally, ACPI sometimes needs to register its own acpi_video0 backlight even if one already exists (such as intel_backlight), which can be done by adding one of the following kernel parameters:
acpi_backlight=video acpi_backlight=vendor acpi_backlight=native
If you find that changing the acpi_video0 backlight does not actually change the brightness, you may need to use acpi_backlight=none.
- On Nvidia Optimus laptops, the kernel parameter
nomodesetcan interfere with the ability to adjust the backlight. - On an Asus notebooks you might also need to load the
asus-nb-wmikernel module. - Disabling legacy boot on Dell XPS13 breaks backlight support.
Udev rule
If the ACPI interface is available, the backlight level can be set at boot using a udev rule:
/etc/udev/rules.d/81-backlight.rules
# Set backlight level to 8
SUBSYSTEM=="backlight", ACTION=="add", KERNEL=="acpi_video0", ATTR{brightness}="8"
setpci
In some cases (e.g. Intel Mobile 945GME [1]), it is possible to set the register of the graphic card to adjust the backlight. It means you adjust the backlight by manipulating the hardware directly, which can be risky and generally is not a good idea. Not all of the graphic cards support this method.
When using this method, you need to use lspci first to find out where your graphic card is.
# setpci -s 00:02.0 F4.B=0
External monitors
DDC/CI (Display Data Channel Command Interface) can be used to communicate with external monitors implementing MCCS (Monitor Control Command Set) over I2C. DDC can control brightness, contrast, inputs, etc on supported monitors. Settings available via the OSD (On-Screen Display) panel can usually also be managed via DDC. The kernel module i2c-dev may need to be loaded if the /dev/i2c-* devices do not exist.
ddcutil can be used to query and set brightness settings:
# ddcutil capabilities | grep "Feature: 10"
Feature: 10 (Brightness)
# ddcutil getvcp 10
VCP code 0x10 (Brightness ): current value = 60, max value = 100
# ddcutil setvcp 10 70
Alternatively, one may use ddcci-driver-linux-dkmsAUR to expose external monitors in sysfs. Then, after loading the ddcci kernel module, one can use any backlight utility.
- Using
ddcciandi2c-devsimultaneously may result in resource conflicts such as aDevice or resource busyerror. - Users of NVIDIA's proprietary drivers may need to add
Option "RegistryDwords" "RMUseSwI2c=0x01; RMI2cSpeed=100"to theDevicesection in/etc/X11/xorg.conf.d/20-nvidia.conf(generated by nvidia-xconfig) oroptions nvidia NVreg_RegistryDwords=RMUseSwI2c=0x01;RMI2cSpeed=100to/etc/modprobe/conf.d/nvidia.conf. See [2] and [3] -
ddcutilwill fail to set some VCP features if there is a feature enabled on the monitor which already automatically adjusts them (e.g. Dynamic Contrast Ratio or BenQ's Eye Care technology). - To facilitate binding screen brightness control to a keyboard shortcut, it may be convenient to enable non-superuser access to the relevant I2C devices. This can be achieved by adding a group
i2cand configuring udev to set this group as the owner of the I2C devices. See [4]. - If ddcutil is installed, it provides the
/usr/share/ddcutil/data/90-nvidia-i2c.conffile, which can be copied to/etc/X11/xorg.conf.d/instead of manually editing Xorg configuration files. It also provides/usr/share/ddcutil/data/45-ddcutil-i2c.rulesand/usr/share/ddcutil/data/45-ddcutil-usb.rulesfor udev rules.
Switch off the backlight
Switching off the backlight (for example when one locks a notebook) can be useful to conserve battery energy. Ideally the following command should work for any Xorg graphical session:
$ xset dpms force off
The backlight should switch on again on mouse movement or keyboard input. Alternately, xset s could be used for a similar effect.
If the previous commands do not work, there is a chance that vbetool may work. Note, however, that in this case the backlight must be manually activated again. The command is as follows:
$ vbetool dpms off
To activate the backlight again:
$ vbetool dpms on
For example, this can be put to use when closing the notebook lid using acpid.
Save and restore functionality
The systemd package includes the service systemd-backlight@.service, which is enabled by default and "static". It saves the backlight brightness level at shutdown and restores it at boot. The service uses the ACPI method described in #ACPI, generating services for each folder found in /sys/class/backlight/. For example, if there is a folder named acpi_video0, it generates a service called systemd-backlight@backlight:acpi_video0.service. When using other methods of setting the backlight at boot, it is recommended to stop systemd-backlight from restoring the backlight by setting the kernel parameters parameter systemd.restore_state=0. See systemd-backlight@.service(8) for details.
systemd-backlight@backlight:acpi_video1 for acpi_video1).Additionally, the brilloAUR and light utilities support save and restore functionality. These two may be more useful if one wishes to restore the screen brightness on a per-user basis, however no systemd units are provided to accomplish this.
Backlight utilities
The utilities in the following table can be used to control screen brightness. All of them are compatible with Wayland and do not require X. Some (like brightnessctl or light) add udev rules to allow members of the video (or input) group to modify brightness.
| Package name | Controls keyboard backlights | Reacts to ambient brightness | Language | License | Notes |
|---|---|---|---|---|---|
| acpilight | Yes | No | Python3 | GPL-3.0-or-later | "xbacklight" executable provided |
| backlight_controlAUR | No | No | C | MIT | Extremely small and simple. Supports relative adjustments. |
| blightAUR | Yes | No | Python3 | ISC | Uses logind interface. Restricted to local users, but does not require suid or video group membership. |
| brightdAUR | No | No | C | GPL-2.0 | Dims the screen when there is no user input for some time. |
| brightnessctl | Yes | No | C | MIT | - |
| brilloAUR | Yes | No | C | GPL-3.0-only | Supports smooth and relative adjustments. |
| clightAUR | Yes | Yes | C | GPL-3.0-or-later | Manages screen temperature (Xorg only) and smoothly dims brightness after a timeout. Supports ambient light sensors [5]. Can turn webcam into an ambient light sensor. |
| enlighten-gitAUR | Yes | No | C | GPL-3.0-or-later | - |
| illum-gitAUR | No | No | C | AGPL-3.0 | Reacts to key presses. |
| light | Yes | No | C | GPL-3.0-only | - |
| luxAUR | No | No | Shell | MIT | - |
| macbook-lighterAUR | Yes | Yes | Bash,Perl | GPL | Macbook screen/keyboard backlight CLI and auto-adjust on ambient light. |
| wlr-brightness-gitAUR | No | No | C | MIT | Also supports newer OLED displays that need gamma adjustment. Uses wlroots. |
| ybacklightAUR | No | No | Perl | GPL-2.0 | Small Perl script similar to xbacklight but using sysfs drivers. |
XF86MonBrightnessUp and XF86MonBrightnessDown keyboard keys as described in Keyboard shortcuts#Xorg.xbacklight
Brightness can be set using the xorg-xbacklight package.
To set brightness to 50% of maximum:
$ xbacklight -set 50
Increments can be used instead of absolute values, for example to increase or decrease brightness by 10%:
$ xbacklight -inc 10 $ xbacklight -dec 10
If you get the "No outputs have backlight property" error, it is because xrandr/xbacklight does not choose the right directory in /sys/class/backlight. You can specify the directory by setting the Backlight option of the device section in /etc/X11/xorg.conf.d/20-video.conf. For instance, if the name of the directory is intel_backlight and using the Intel driver, the device section may be configured as follows:
/etc/X11/xorg.conf.d/20-intel.conf
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "Backlight" "intel_backlight"
EndSection
See FS#27677 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=651741 for details.
If you have enabled Intel Fastboot you might also get the No outputs have backlight property error. In this case, trying the above method may cause Xorg to crash on start up. You should disable it to fix the issue. It is known to cause issues with brightness control.
Using DBus with GNOME
Brightness can also be adjusted as the GNOME controls do. Changes are reflected in the GNOME UI using this method.
$ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.freedesktop.DBus.Properties.Set org.gnome.SettingsDaemon.Power.Screen Brightness "<int32 50>"
Steps in brightness for keyboard control can be implemented with this method as well.
$ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp $ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepDown
Using DBus with KDE
See https://userbase.kde.org/KDE_Connect/Tutorials/Useful_commands#Brightness_settings.
Color correction
Color correction does not change the backlight power, it just modifies the video lookup table: this means that your battery life will be unaffected by the change. Nevertheless, it could be useful when no backlight control is available (desktop PCs or laptops with OLED screens).
- Clight — User daemon utility that aims to fully manage your display. It can manage the screen temperature depending on the current time of the day, just like redshift does. It tries to use geoclue to retrieve the user position if neither latitude or longitude are set in the configuration file. It also supports fixed times for sunrise and sunset.
- icc-brightness — Control OLED display brightness by applying ICC color profiles.
- Monica — Monitor calibration tool. It works as frontend to xgamma to alter the gamma correction.
- https://web.archive.org/web/20090815224839/http://www.pcbypaul.com/software/monica.html || monicaAUR
- Redshift — Color temperature adjustment tool. It adjusts the color temperature of your screen according to your surroundings. This may help your eyes hurt less if you are working in front of the screen at night. This program is inspired by f.lux.
- xcalib — Lightweight monitor calibration loader which can load an ICC monitor profile to be shared across desktop applications.
- xgamma — Alter a monitor's gamma correction.
Wayland
Redshift does not support Wayland (without a patch or fork like redshift-wayland-gitAUR). But it is possible to apply the desired temperature in tty before starting a compositor. For example:
$ redshift -m drm -PO 3000
Otherwise some compositors can apply color correction during runtime:
- On GNOME, the built-in Night Light can be used.
- On KDE Plasma, the built-in KDE#Night Color can be used.
- On Sway 1.0 and other wlroots-based compositors, as well as Orbital, Redshift fork gammastep, clightAUR, or wlsunset-gitAUR can be used.
Xorg: adjust perceived brightness with xrandr
xrandr may be used to adjust the perceived brightness.
To adjust perceived brightness above its maximum level (the same caveats mentioned above for Nvidia apply):
$ xrandr --output output_name --brightness 2
This should roughly double luma in the image. It will sacrifice color quality for brightness, nevertheless it is particularly suited for situations where the ambient light is very bright (e.g. sunlight).
This can also be used to reduce perceived brightness in a dark room by specifying some value less than 1 (e.g. 0.5), this is useful when no backlight control is available (e.g. desktop PC).
The output name of the connected device may be determined by calling xrandr:
$ xrandr | grep -w connected | cut -f '1' -d ' '
Users may find it convenient to implement this as an alias:
$ alias b='echo -e "enter brightness:\n"; read val; xrandr --output output name --brightness "${val}"'
To automatically call xrandr when a backlight file changes, oled_shmoledAUR can be used like so:
$ oled_shmoled output_name
NVIDIA settings
Users of NVIDIA's proprietary drivers can change display brightness via the nvidia-settings utility under "X Server Color Correction." However, note that this has absolutely nothing to do with backlight (intensity), it merely adjusts the color output. (Reducing brightness this way is a power-inefficient last resort when all other options fail; increasing brightness spoils your color output completely, in a way similar to overexposed photos.)
Troubleshooting
Backlight PWM modulation frequency (Intel i915 only)
Laptops with LED backlight are known to have screen flicker sometimes. This is because the most efficient way of controlling LED backlight brightness is by turning the LED's on and off very quickly varying the amount of time they are on.
However, the frequency of the switching, so-called PWM (pulse-width modulation) frequency, may not be high enough for the eye to perceive it as a single brightness and instead see flickering. This causes some people to have symptoms such as headaches and eyestrain.
If you have an Intel i915 GPU, then it may be possible to adjust PWM frequency to eliminate flicker.
Period of PWM (inverse to frequency) is stored in 2 higher bytes of 0xC8254 register (if you are using the Intel GM45 chipset use address 0x61254 instead). To manipulate registers values install intel-gpu-tools from the official repositories.
To increase the frequency, period must be reduced. For example:
# intel_reg read 0xC8254
0xC8254 : 0x12281228
Then to double PWM frequency divide 2 higher bytes (4 higher hex digits) by 2 and write back resulting value, keeping lower bytes unchanged:
# intel_reg write 0xC8254 0x09141228
You can use online calculator to calculate desired value https://devbraindom.blogspot.com/2013/03/eliminate-led-screen-flicker-with-intel.html
To set new frequency automatically, consider writing an udev rule or install intelpwm-udevAUR.
Inverted Brightness (Intel i915 only)
Symptoms:
- after installing xf86-video-intel systemd-backlight.service turns off the backlight during boot
- possible solution: mask systemd-backlight.service
- switching from X to another VT turns the backlight off
- the brightness keys are inverted (i.e. turning up the brightness makes the screen darker)
This problem may be solved by adding i915.invert_brightness=1 to the list of kernel parameters.
Unable to control eDP Panel brightness (Intel i915 only)
Embedded Display Port (eDP) v1.2 introduced a new display panel control protocol for backlight and other controls that works through the AUX channel [7]
By default the i915 driver tries to use PWM to control backlight brightness, which might not work.
To set the backlight through writes to DPCD registers using the AUX channel set i915.enable_dpcd_backlight=1 as a kernel parameter.
sysfs modified but no brightness change
On some systems, the brightness hotkeys on your keyboard correctly modify the values of the acpi interface in /sys/class/backlight/acpi_video0/actual_brightness but the brightness of the screen is not changed. Brightness applets from desktop environments may also show changes to no effect.
If you have tested the recommended kernel parameters and only xbacklight works, then you may be facing an incompatibility between your BIOS and kernel driver.
In this case the only solution is to wait for a fix either from the BIOS or GPU driver manufacturer.
A workaround is to use the inotify kernel api to trigger xbacklight each time the value of /sys/class/backlight/acpi_video0/actual_brightness changes.
First install inotify-tools. Then create a script around inotify that will be launched upon each boot or through autostart.
/usr/local/bin/xbacklightmon
#!/bin/sh
path=/sys/class/backlight/acpi_video0
luminance() {
read -r level < "$path"/actual_brightness
factor=$((100 / max))
printf '%d\n' "$((level * factor))"
}
read -r max < "$path"/max_brightness
xbacklight -set "$(luminance)"
inotifywait -me modify --format '' "$path"/actual_brightness | while read; do
xbacklight -set "$(luminance)"
done
Backlight not working in MATE
Make sure the mate-power-manager package is installed.
Backlight keys not working in Xfce
In xfce4, the Xfce4 Power Manager handles the brightness keys.
In some installations of Xfce, the "Handle display brightness keys" setting may be turned off by default.
To activate the brightness keys again, open the Xfce Power Manager dialog and toggle on "Handle display brightness keys":
$ xfce4-power-manager -c
xbacklight returns : No outputs have backlight property
Depending on the video card installed, sometimes xbacklight from xorg-xbacklight returns the message "No outputs have backlight property". Installing acpilight provides an alternative xbacklight that may work as expected.
Backlight is always at full brightness after a reboot with amdgpu driver
Due to a bug introduced recently in the amdgpu driver, the backlight's actual_brightness value is reported as a 16-bit integer, which is outside the 8-bit range specified in max_brightness. This causes the systemd-backlight service to attempt to restore, at boot time, a value that is too large and ends being truncated to maximum brightness (255).
While the bug is not addressed, one possible workaround is to modify the stored brightness to within the correct range before it's restored. This can be accomplished with a script and a service unit:
fix-brightness.sh
#!/bin/bash
# Change the line below according to your hardware
BRIGHTNESS_FILE="/var/lib/systemd/backlight/pci-0000:04:00.0:backlight:amdgpu_bl0"
BRIGHTNESS=$(cat "$BRIGHTNESS_FILE")
BRIGHTNESS=$(($BRIGHTNESS*255/65535))
BRIGHTNESS=${BRIGHTNESS/.*} # truncating to int, just in case
echo $BRIGHTNESS > "$BRIGHTNESS_FILE"
fix-brightness.service
[Unit] Description=Convert 16-bit brightness values to 8-bit before systemd-backlight applies it Before=systemd-backlight@backlight:amdgpu_bl0.service [Service] Type=oneshot ExecStart=<path to the script above> [Install] WantedBy=multi-user.target
On certain systems, the backlight level reported by the driver is in the correct range [0, 255], but systemd still fails to restore the correct value. This is probably due to a race in the kernel. In this case, truncating the brightness level will not help since it's already in the correct range. Instead, saving the brightness level to systemd before shutting down could work as a workaround. This can be accomplished by the following script and service unit:
fix-brightness.sh
#!/bin/sh # Backlight level from systemd's perspective (change if needed) readonly SYSTEMD_BACKLIGHT_FILE='/var/lib/systemd/backlight/pci-0000:04:00.0:backlight:amdgpu_bl0' # Backlight level from AMDGPU driver readonly AMDGPU_BACKLIGHT_FILE='/sys/class/backlight/amdgpu_bl0/brightness' # Read current value from the driver and apply it to systemd readonly AMDGPU_BACKLIGHT_VALUE=$(cat "$AMDGPU_BACKLIGHT_FILE") echo "$AMDGPU_BACKLIGHT_VALUE" > "$SYSTEMD_BACKLIGHT_FILE"
fix-brightness.service
[Unit] Description=Save brightness value from AMDGPU DefaultDependencies=no After=final.target [Service] Type=oneshot ExecStart=<path to the script above> [Install] WantedBy=final.target