if you *cannot* use the fn key on a laptop to change the screen backlight (brightness) - see: http://community.linuxmint.com/tutorial/view/1300  -  for a hardware setting.

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if you *can* use the fn key on a laptop to change the screen backlight, but you wish to change the brightness of the screen on login, then you can use a software setting:

1. make sure xbacklight is installed - enter: xbacklight

2. to install enter: sudo aptitude install xbacklight

3. (on linux mint 16 petra) edit the default mdm startup file: sudo gedit /etc/mdm/Init/Default

4. add this line to the end of Default: xbacklight =20

5. save the file and reboot the laptop.

*note: in step 4 I assume that xbacklight is already in the $PATH. You can change the % (20) to any other value.

*note:  the idea is to add xbacklight command to the default startup file of the display manager on your system. so even if you are using another display manager, or another default file - the proccess is the same, only the file path is different.

that's all.