Let's first install xbacklight with this command:
sudo apt-get install xbacklight
Open "Startup Applications" and add the following details:
- Name: Brightness
- Command: xbacklight -set 60
On the next reboot, your system will set the selected brightness level automatically after you log in.
NOTE: This tip is workable under Ubuntu 12.10/12.04/11.10 and Linux Mint 14/13.