Cannot adjust brightness after sleep

Solution 1:

I found a better solution than the one selected above.

1. Press Alt + F2 and type gksudo gedit /etc/default/grub.
2. Find the line than starts with GRUB_CMDLINE_LINUX_DEFAULT= and modify it as follows: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor".
3. In a terminal (ctrl + alt + T) type sudo update-grub.
4. Reboot.

Problem solved.

More info here: https://wiki.ubuntu.com/Kernel/Debugging/Backlight

Edit

This solution no longer works on 13.10 and 14.04.

A solution that does is creating an xorg configuration file:

1. In a terminal type sudo touch /usr/share/X11/xorg.conf.d/20-intel.conf to create the file.
2. Followed by sudo gedit /usr/share/X11/xorg.conf.d/20-intel.conf to edit its content.
3. Then add the following content and save:

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

EndSection

Log out and in again.

Source: Fix Brightness Control Not Working for Ubuntu 13.10 & 14.04

Solution 2:

I had the same problem with a Toshiba Portege R935 in 12.04, and I discovered this (not so elegant) workaround:

• as root (not with sudo but actually changing user: su root), change the value in sys/class/backlight/intel_backlight/brightness

  for instance:

  echo 2937 > /sys/class/backlight/intel_backlight/brightness
  

Warning: these values are not between 0 and 7 (as the ones in the other directory), but between 284 and 4539, or something like that.