I cannot change the screen brightness

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