I was always unable to change my brightness since I installed Ubuntu, but I figured that installing the latest ATI driver for my graphics card would work.
I did, but I still can't change the brightness. The slider goes up and down, but the brightness stays the same (on full).
I have switchable graphics, an ATI Radeon HD 6770M, and an Intel integrated GPU.
To get working brightness key. try following
press Alt+f2 , execute gksudo gedit /etc/default/grub
Change this line
GRUB_CMLINE_LINUX_DEFAULT="quiet splash"
to something like below
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Open a terminal by Alt+ctrl +T, Execute sudo update-grub and reboot . See if brightness keys are working.
If the above is not working set a fixed value for brightness. To do so,
Open a terminal by pressing Alt+ctrl+T.
Type ls /sys/class/backlight/*/brightness, copy the outputs to a separate text file
The possibility is there will be two paths,try the following for each path and replace accordingly
example paths will be like
/sys/class/backlight/acpi_video0/brightness
Run following commands in terminal,
For the above path Get the maximum brightness:
cat /sys/class/backlight/acpi_video0/max_brightness`
Try a lower value to set the brightness, say output is 16 so I will try with half of it
echo 8 | sudo tee /sys/class/backlight/acpi_video0/brightness
If this works, make this happen in each login automatically by doing the following
sudo gedit /etc/rc.local
Enter this line just before exit 0. It should look like
echo YOUR_VALUE_HERE > /sys/class/backlight/acpi_video0/brightness
exit 0
Also you can try with xdotool 
