I faced this problem when I bought a new HP Laptop, hoping to install Ubuntu, but when I booted from a Live DVD the brightness was set at max and couldn't be changed with the function keys or from the brightness settings bar, and it was the same after installation.
After a lot of searching on the net, I tried the acpi_backlight=vendor kernel option, and it worked! Not only for Ubuntu, but for other Linux OSes as well, so I am posting a guide.
Restart your system, and when you get to the GRUB menu, select your distribution (Ubuntu 13.04), don't press enter, but instead press e (to edit).
This will bring up your menu entry for editing.
Append to the line with starts with linux (and looks like linux /boot/linux3.9. ............
At the end of this line, leave a space and add acpi_backlight=vendor
so that it looks like
linux /boot/vmlinuz-310-x86_64 root=UUID=3161dd63-38c0-44bc-bc1e-34b692209b0b ro acpi_backlight=vendor
After this press F10 to boot.
Now try changing the brightness. It works for me.
To make the change permanent we need to edit the GRUB configuration file
In a terminal type
sudo -H gedit /etc/default/grub
This will open the Grub configuration file.
In this file we have to edit the line
GRUB_CMDLINE_LINUX=""
to
GRUB_CMDLINE_LINUX="acpi_backlight=vendor"
After that save the file and run (type in the terminal)
sudo update-grub
to update the GRUB configuration file.
That's It! Now the brightness settings can be changed any time and will be persistent across reboots (at least for me!)
If anyone has queries please post them here.
Edit
Since Linux 3.16, the kernel by default uses the native backlight method, so the above steps could be redundant.
Source: https://wiki.archlinux.org/index.php/Backlight#Kernel_command-line_options
