Brightness problem Ubuntu 18.04 LTS

I also had this problem, but on different distributions the solution was different.

Solution provided by Anas Elazhar worked well until I switched to Xubuntu:

Open the file /etc/default/grub using gedit or any other text editor. Find below line.

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"  

Change above line to

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

If it doesn't work for you either try to change acpi_backlight value to video:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=video"

Also create the file /usr/share/X11/xorg.conf.d/80-backlight.conf but content is slightly different from the file that you created:

Section "Device"
    Identifier  "Intel Graphics"
    Driver      "intel"
    Option      "AccelMethod"     "sna"
    Option      "Backlight"       "acpi_video0"
    BusID       "PCI:0:2:0"
EndSection

If combination acpi_backlight=video with this file doesn't work change acpi_backlight back to vendor.

And do not forget to run sudo update-grub command in terminal every time you change /etc/default/grub!

Open the file /etc/default/grub using gedit or any other text editor. Find below line.

  GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"  

Change above line to

  GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

This would ensure to load device specific drivers before default drivers in Linux.

Save the file and close text editor.

Open terminal and run below command to update grub.

  sudo update-grub

Reboot.

After reboot, try adjusting brightness using Laptop dedicated control keys. Good Luck.

Installing Brightness Controller helped me in fixing up the issue

 sudo add-apt-repository ppa:apandada1/brightness-controller
 sudo apt update
 sudo apt install brightness-controller

You can access the Brightness Controller after download by searching for it.