In some cases (e.g. Intel Mobile 945GME [1]), it is possible to set the register of the graphics 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 graphics cards support this method.

The backlight should switch on again on mouse movement or keyboard input. Alternately, xset s could be used for a similar effect.

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/AMD 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:

There are many ways to control brightness of a monitor, laptop or integrated panel (such as the iMac). According to these discussions[dead link 2024-10-12 ⓘ] and this wiki page the control method can be divided into these categories:

To activate the brightness keys again, open the Xfce Power Manager dialog and toggle on "Handle display brightness keys":

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[dead link 2024-10-12 ⓘ] to cause issues with brightness control.

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.

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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 the intel-gpu-tools package.

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.

To set the backlight through writes to DPCD registers using the AUX channel set i915.enable_dpcd_backlight=1 as a kernel parameter.

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.

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 (Logging out/Rebooting may be necessary to changes take effects):

While the bug is not addressed, one possible workaround is to modify the stored brightness to within the correct range before it is restored. This can be accomplished with a script and a service unit:

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:

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).

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 is 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:

You can use online calculator to calculate desired value https://devbraindom.blogspot.com/2013/03/eliminate-led-screen-flicker-with-intel.html

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.)

Additionally, the brilloAUR and lightAUR 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.

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.

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).

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.

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.

Redshift does not support Wayland (although a patchset exists). But it is possible to apply the desired temperature in tty before starting a compositor. For example:

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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.

Display Data Channel Command Interface (DDC/CI) can be used to communicate with external monitors implementing Monitor Control Command Set (MCCS) over I2C. DDC can control brightness, contrast, inputs, etc on supported monitors. Settings available via the On-Screen Display (OSD) 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.

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:

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).

According to systemd-backlight@.service(8), if the udev property ID_BACKLIGHT_CLAMP is not set to false, the brightness is clamped to a value of at least 1 or 5% of maximum brightness, whichever is greater. This restriction will be removed when the kernel allows user space to reliably set a brightness value which does not turn off the display.

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 brightness can be set by writing a number to brightness. Attempting to set a brightness greater than the maximum results in an error.

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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/.

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).

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.

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]

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:

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.

First install inotify-tools. Then create a script around inotify that will be launched upon each boot or through autostart.

Then to double PWM frequency divide 2 higher bytes (4 higher hex digits) by 2 and write back resulting value, keeping lower bytes unchanged: