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 Os 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), dont press enter, but press e (to edit).
This would 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)
so that it will look 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.
Now we have to make it permanent
To do that we need to edit the Grub configuration file-
In a terminal type
sudo gedit /etc/default/grub
This would open the Grub configuration file.
In this file we have to edit the line
After that save the file and run (type in the terminal)
to update the Grub configuration file.
Thats It! Now the brigtness settings can be changed any time and will be persistent across reboots (at least for me!)
If anyone has queries please post them here.
Since linux 3.16, the kernel by default uses the native backlight method, so the above steps could be redundant.