Try this:

1. Open a terminal (Ctrl + Alt + T).


2. Then type sudo nano /etc/default/grub. It will ask for your password. Type it in.


3. Around the 11th line, there will be something like: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash". Change it to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
   


4. Save the file by Ctrl+O followed by Ctrl+X. Then run sudo update-grub in the terminal.


5. Reboot and see if backlight adjustment works. If not, undo the changes you did above, by invoking the text editor as in steps 1 and 2.

Hope it helps.

Works for Acer Aspire v3-571,Acer Aspire v3 571g,Hewlett Packard Bell EasyNote TS,Acer Aspire 4755G,Acer Aspire 5750-6866, Acer Aspire 5739, Lenovo T540p

OP reported in Revisions 2 & 3 of the question that the following worked for him.

I figured it out from different sites, it fixes backlight.

Run the following command in Terminal:

gksu gedit /etc/default/grub

then change

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

GRUB_CMDLINE_LINUX=""

to

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

GRUB_CMDLINE_LINUX="acpi_osi=Linux"

then save and run:

sudo update-grub 

and then restart the system for changes to take effect.

Ubuntu 14.04 (13.10+) with intel graphics

How to check if graphics card is intel

First, check if your graphics card is intel. You can check it from System Settings->Details->Graphics or with following command:

ls /sys/class/backlight

You should see something like:

ideapad  intel_backlight

Fix backlight

Make sure /usr/share/X11/xorg.conf.d/20-intel.conf exists. If it doesn't, make it yourself and add the following:

Section "Device"

        Identifier  "card0"

        Driver      "intel"

        Option      "Backlight"  "intel_backlight"

        BusID       "PCI:0:2:0"



EndSection

Logout and Login. Done.

Thanks to Abhishek

Reposted a solution that worked for me http://itsfoss.com/fix-brightness-ubuntu-1310/

[Like in richy's, but] I use GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi="

(I'm on an Aspire 5750-6866.) It works fine, same control levels I had in Windows.

My brightness used to be stuck at max before I discovered this.

(Although brightness levels still reset after a restart/hibernate/shutdown)

I found out a solution that worked with my laptop:

add this to /etc/rc.local:

echo 2 > /sys/class/backlight/acpi_video0/brightness

I had the same problem.
I have a Travelmate P633-V and I did this code to fix it.

After installing the scripts my FN-Left and FN-Right are working fine.

The script should work with all intel devices that exposes
/sys/class/backlight/intel_backlight

https://github.com/codingtony/acer-brightness-linux-acpi

I got this problem when i upgraded from 11.10 to 12.04.
enter this in terminal
code:

sudo gedit /etc/default/grub

Check for these lines GRUB_CMDLINE_LINUX_DEFAULT and change it as the below and save it.

After the update the grub

code: sudo update-grub

Code:

GRUB_DEFAULT=0

GRUB_HIDDEN_TIMEOUT=0

GRUB_HIDDEN_TIMEOUT_QUIET=true

GRUB_TIMEOUT=10

GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`

`**GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset acpi_backlight=vendor"**`

GRUB_CMDLINE_LINUX=""



# Uncomment to enable BadRAM filtering, modify to suit your needs

# This works with Linux (no patch required) and with any kernel that obtains

# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)

#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"



# Uncomment to disable graphical terminal (grub-pc only)

#GRUB_TERMINAL=console

# The resolution used on graphical terminal

# note that you can use only modes which your graphic card supports via VBE

# you can see them in real GRUB with the command `vbeinfo'

#GRUB_GFXMODE=640x480



# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux

#GRUB_DISABLE_LINUX_UUID=true



# Uncomment to disable generation of recovery mode menu entries

#GRUB_DISABLE_RECOVERY="true"



# Uncomment to get a beep at grub start

#GRUB_INIT_TUNE="480 440 1" 

Most simple and fast way to do this....
Use this command

xrandr --output LVDS1 --brightness 0.5

Set the value in between 0 to 1 like in this case it is 0.5
This works for me
Try this !!!

Whilst trying xbacklight didn't work for me because I'm using NVIDIA drivers, Light did the job pretty well for me.

After installing:

• 
  

  Increase backlight brightness by 5 percent

  
  
  

  light -A 5
  
  

  
  


• 
  

  Decrease backlight brightness by 5 percent

  
  
  

  light -U 5
  
  

  
  

Try the following:

sudo gedit /etc/default/grub

Then change this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor"
Then in a terminal type sudo update-grub
Reboot and see if the problem is solved.

(Defunct) solution for Ubuntu 12.04.1:

Use Add Drivers / Additional Drivers to load Cedar Trail drm driver (closed source).

For Ubuntu 12.04.2 (and fully updated 12.04) this issue is resolved by more recent updates. It does not require the proprietary driver.

If you have just done a fresh install of 12.04.2, then you need to update (and re-start) to fix this.

At terminal, type sudo apt-get update && sudo apt-get upgrade and then sudo apt-get dist-upgrade.

Update: (28-Apr-2013)

Repeated steps, with fresh install. This did not fix problem.

While the brightness is fixed, any dimness/brightness adjustment (using function keys) does not seem work after this fix ..

Ongoing ...

I was struggling with this problem as well. My notebook is an Acer Aspire E1-522. I could solve the brightness issue by changing from the X.Org X server to the proprietary AMD video driver.

To do so, go to Software & Updates and then go to the tab Additional Drivers. There you can probably find the proprietary driver. Select it and click Apply Changes. You will need to reboot you computer in order to know if it really helped.

On Acer Aspire 4740 after installing Ubuntu 18.04, Screen brightness would not change.
Tried every thing above, did not help.

Added blacklist acer-wmi to /etc/modprobe.d/blacklist.conf and rebooted

The Fn shortcut started working.

Reference:
https://ubuntuforums.org/archive/index.php/t-2172282.html

Same solution as one-liner

For this solution, no nano knowledge is required. As such, it may also come handy for multi-machine installation scripts.

sudo sed -i 's|^GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"|GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"|' /etc/default/grub && sudo update-grub

For the faint of heart, the above command edits the file /etc/default/grub to replace the appropriate line with GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

After editing, a sudo update-grub should be issued for the changes to take effect.