Difference between revisions of "Backlight"

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  redshift -o -l 0:0 -b 0.8 -t 6500:6500
 
  redshift -o -l 0:0 -b 0.8 -t 6500:6500
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{{Tip|If your longitude is west or your latitude is south, you should input it as negative.
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Example for Berkeley, CA:
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gtk-redshift -l 37.8717:-122.2728
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}}
  
 
===relight===
 
===relight===
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== NVIDIA Settings ==
 
== NVIDIA Settings ==
 
Users of [[NVIDIA|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.)
 
Users of [[NVIDIA|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 instance, to double frequency from the previous listing, execute
 +
 +
# intel_reg_write 0xC8254 0x09140914
 +
 +
Refer to dedicated topic for details https://bbs.archlinux.org/viewtopic.php?pid=1245913

Revision as of 04:27, 30 March 2013

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 category:

  • 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 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 is preferred by xbacklight as default. See FS#27677 for xbacklight, if you get "No outputs have backlight property"

  • 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 screens 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 issue 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.

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 program xcalib can be downloaded from AUR and 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 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.

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's 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.

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 instance, to double frequency from the previous listing, execute

# intel_reg_write 0xC8254 0x09140914

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