here's my problem:

Ever since I installed Ubuntu 12.10 on my computer I haven't been able to change the screen brightness, it was always set to the maximum. I did take a look at this post:

Decrease Backlight Below Minimum

And it helped me resolved the issue with the unbearable brightness, however one problem persists: I cannot change it through "Brightness & Lock" or the function buttons (F2 and F3), when I press the buttons, the animation shows up but nothing happens.
I tried the following:

http://www.refreshit.info/2012/08/solved-brightness-increase-and-decrease.html

It did nothing...

If it helps:

I have an HP-dv6 6185la, it has an ATI Radeon 6770M HD card and an Intel HD Integrated Graphics Card

It only detects the "Intel® Sandybridge Mobile"

Thanks in advance.

What worked for me was to change the /etc/default/grub file as follows:

• Open a terminal (Ctrl + Alt + T) and type sudo gedit /etc/default/grub



• 
  

  Find the line: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" and modify it to one of the following:

  
  
  

  GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

  
  
  

  or

  
  
  

  GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux
acpi_backlight=vendor" (try this if the first one doesn't work)

  
  



• Save and close gedit.



• Then type sudo update-grub
  


• Reboot your pc.

EDIT

As from Ubuntu 13.10, this did not solve the problem for me on a Toshiba laptop. I needed one extra step for the problem to be solved.

I had to create an xorg.conf file in /etc/X11/ with the following contents:

Section "Device"
 Identifier "Configured Video Device"
 Driver "intel"
 Option "Backlight" "intel_backlight"
EndSection
#
Section "Monitor"
 Identifier "Configured Monitor"
EndSection
#
Section "Screen"
 Identifier "Default Screen"
 Monitor "Configured Monitor"
 Device "Configured Video Device"
EndSection

WARNING!! Getting the file wrong or if this configuration is not compatible with your hardware, might end up with an unbootable system! It happened to me.

In that case, you'll have to delete (or rename) the file, but to do so requires either booting from a liveUSB or liveDVD (the simplest way) or booting in recovery mode, making the Ubuntu partition rewritable and then delete the offending xorg.conf file.

EDIT2

Starting with kernel 4.4, the toshiba backlight device is blacklisted and doesn't appear any longer thus rendering these workarounds redundant.

I tried several solutions, including changing grub, xbacklight, and several others. The method that works for me is changing NVIDIA settings.

1. In terminal, gksudo gedit /etc/X11/xorg.conf
   


2. Add line Option "RegistryDwords" "EnableBrightnessControl=1" immediately above EndSection
   


3. Save and reboot

Here is the original page:

https://wiki.archlinux.org/index.php/NVIDIA#No_brightness_control_on_laptops

This solved my problem of "brightness & lock not working for changing the brightness" on Acer Aspire 5755 Ubuntu 15.04.
add the following lines in the file:/usr/share/X11/xorg.conf.d/20-intel.conf

Section "Device"

 Identifier "card0"
 Driver "intel"
 Option "Backlight" "intel_backlight"
 BusID "PCI:0:2:0"

EndSection

THE REVISION for Ubuntu 17.10 , Based on this bug report. Seems little bit same issue. Follow the Developers Guide :

1. 
   

   Check the module which exactly controlling the brightness are acpi_video0 or intel_backlight.

   
   
   

   $ ls /sys/class/backlight/

   
   


2. 
   

   Run tee /sys/class/backlightt/acpi_video0/brightness <<< 5

   
   
   

   If nothing happens, then intel_backlight is the one handling the brightness settings.

   
   


3. 
   

   Next step is modifying the file /etc/default/grub include four lines.

   
   
   

   GRUB_CMDLINE_LINUX="acpi_backlight=none"
GRUB_CMDLINE_LINUX="acpi_backlight=video"
GRUB_CMDLINE_LINUX="acpi_backlight=vendor"
GRUB_CMDLINE_LINUX="acpi_backlight=native"

   
   



4. Update the grub $sudo update-grub and restart your system.



5. 
   

   The Verification.

   
   
   

   $ ls /sys/class/backlight/

   
   

Voila!!!! acpi_video0 is gone!
Check the fn+Brightness keys or change the brightness via the applet. Brightness controls should work just fine.

Note: The Guide already on this comment #9.

Try this, it helped me with intel gpu
http://ubuntufixer.blogspot.com/2012/12/set-screen-brightness-at-start-up.html