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# Backlight

From ArchWiki

Screen brightness can often be tricky to control. On many machines, physical hardware switches are missing and software solutions may or may not work well. Make sure to find a working method for your hardware! Too bright screens can cause eye strain.

There are many ways to adjust the screen backlight of a monitor, laptop or integrated panel (such as the iMac) using software, but depending on hardware and model, sometimes only some options are available. This article aims to summarize all possible ways to adjust the backlight.

## Overview

There are many ways to control brightness. According to this discussion[1] and this wiki page [2], the control method could be divided into these categories:

- brightness is controlled by vendor specified hotkey. And there is no interface for OS to adjust brightness.
- brightness is controlled by OS:
  - brightness could be controlled by ACPI
  - brightness could be controlled by graphic driver.

All methods expose themselves to the user by /sys/class/brightness. And xrandr/xbacklight could use this folder and choose one method to control brightness. But it is still not very clear which one xbacklight prefers by default. See FS#27677 for xbacklight, if you get "No outputs have backlight property." There is a temporary fix if xrandr/xbacklight does not choose the right directory in /sys/class/brightness: You can specify the one you want in xorg.conf by setting the "Backlight" option of the Device section to the name of that directory (see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=651741 at the bottom of the page for details).

- brightness is controlled by HW register throught setpci

## ACPI

It is often possible to adjust the backlight by ACPI. This controls the actual LEDs or cathodes of the screen. When this ACPI option is available, the illumination is controllable using a GUI slider in the Display/Screen system settings or by simple commands on the CLI.

Different cards might manage this differently. Check `/sys/class/backlight` to find out:

    # ls /sys/class/backlight/

    intel_backlight

So this particular backlight is managed by an Intel card. It is called `acpi_video0` on an ATI card. In the following example, acpi_video0 is used.

The directory contains the following files and folders:

    actual_brightness  brightness         max_brightness     subsystem/    uevent
    bl_power           device/            power/             type

The maximum brightness (often 15) can be found by running `cat`:

    # cat /sys/class/backlight/acpi_video0/max_brightness
    15

Brightness can then be set (as root) with `echo`. Obviously you cannot go any higher than your screen's maximum brightness. The values for maximum brightness and brightness in general vary wildly among cards.

    # echo 5 > /sys/class/backlight/acpi_video0/brightness

Sometimes ACPI does not work well due to different motherboard implementations and ACPI quirks. This include some models with dual graphics (e.g. Nvidia-optimus/Radeon with intel (i915)) and some examples with this problem in notebooks such as Dell Studio, Dell XPS 14/15/17 and some Lenovo series, Kamal Mostafa kernel developer make patches for solved this problem included after 3.1 kernel version. You can try adding the following kernel parameters in your bootloader(grub, syslinux...) to adjust ACPI model:

    acpi_osi=Linux acpi_backlight=vendor

or

    acpi_osi=Linux acpi_backlight=legacy

acpi_backlight=vendor will prefer vendor specific driver (e.g. thinkpad_acpi, sony_acpi, etc.) instead of the ACPI video.ko driver.

Tip: On some new laptops with pre-installed Windows 8 and/or hybrid graphics

You need to add kernel flag to control backlight:

    acpi_osi="!Windows2012"

Tip: Also, you can try:

     acpi_osi="!Windows 2012" acpi_backlight=vendor
     acpi_backlight=legacy
     acpi_osi=Linux

and all combinations of these lines.

The first line works on asus G750 notebook (keys don't work, only from /sys/class/backlight/asus-nb-wmi/brightness. You need to also do

    modprobe asus-nb-wmi

## Switching off the backlight

Switching off the backlight (for example when one locks the notebook) can be useful to conserve battery energy. Ideally the following command inside of a graphical session should work:

    sleep 1 && xset dpms force off

The backlight should switch on again on mouse movement or keyboard input. If the previous command does not work, there is a chance that `vbetool` works. 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 as outlined in the entry for Acipd.

## Backlight utilities

### xbacklight

You can adjust the backlight through the xorg-server command `xbacklight`. The utility is provided by the xorg-xbacklight package in [extra].

A useful demonstration was posted by gotbletu on YouTube. He suggests the following commands to adjust the backlight:

- brighten up:

    xbacklight -inc 40

- dim down:

    xbacklight -dec 40

### xcalib

The package xcalib (upstream url) is available in the AUR and can be used to dim the screen. Again, the user gotbletu posted a demonstration on Youtube. This program can correct gamma, invert colors and reduce contrast, the latter of which we use in this case:

- dim down:

    xcalib -co 40 -a

This program uses ICC technology to interact with X11 and while the screen is dimmed, you may find that the mouse cursor is just as bright as before.

### redshift

The program redshift in the community repository uses `randr` to adjust the screen brightness depending on the time of day and your geographic position. It can also do RGB gamma corrections and set color temperatures. As with `xcalib`, this is very much a software solution and the look of the mouse cursor is unaffected. To execute a single quick adjustment of the brightness, try something like this:

    redshift -o -l 0:0 -b 0.8 -t 6500:6500

Tip: If your longitude is west or your latitude is south, you should input it as negative.

Example for Berkeley, CA:

    gtk-redshift -l 37.8717:-122.2728

### relight

relight is available in Xyne's repos and as package relight in the AUR. The package provides a service to automatically restore previous backlight settings during reboot along using the ACPI method explained above. The package also contains a dialog-based menu for selecting and configuring backlights for different screens.

### setpci (use with great care)

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

### Calise

The software calise can be found in AUR.

- Stable version: calise
- Development version: calise-git

It basically computes ambient brightness, and set screen's correct backlight, simply making captures from the webcam, for laptop without light sensor. For more information, calise has its own wiki: Calise wiki.

The main features of this program are that it is very precise, very light on resource usage, and with the daemon version (.service file for systemd users available too), it has practically no impact on battery life.

### brightd

Macbook-inspired brightd automatically dims (but does not put to standby) the screen when there is no user input for some time. A good companion of Display Power Management Signaling so that the screen does not blank out in a sudden.

## KDE

KDE users can adjust the backlight via System Settings -> Power Management -> Power Profiles. If you want set backlight before kdm just put in /usr/share/config/kdm/Xsetup :

    xbacklight -inc 10

## NVIDIA Settings

Users of NVIDIA's proprietary drivers users 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.)

## Backlight PWM modulation frequency (Intel i915 only)

Laptops with LED backlight are known to have screen flicker sometimes. The reason for this, is that it is hard enough to dim LEDs by limiting direct current flowing through. It is easier to control brightness by switching LEDs on and off fast enough.

However, frequency of the switching (so-called PWM modulation frequency) is not high enough actually, and some people may notice flicker either explicitly or by feeling headache and eyestrain.

If you have an Intel i915 GPU, then it may be possible to adjust PWM modulation frequency to eliminate flicker.

Install intel-gpu-tools from community repo

    # pacman -S intel-gpu-tools

Get value of the register, that determines PWM modulation frequency

    # intel_reg_read 0xC8254
    0xC8254 : 0x12281228

The value returned represents period of PWM modulation. So to increase PWM modulation frequency, value of the register has to be reduced. For example, to double frequency from the previous listing, execute

    # intel_reg_write 0xC8254 0x09140914

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

Refer to dedicated topic for details https://bbs.archlinux.org/viewtopic.php?pid=1245913

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