i insttalled ubuntu 13.10 on my COMPAQ PRESARIO CQ-41 209TU notebook.but whenever it comes on login screen, display backlight turns to 0.why?please help me.
I had this problem on a laptop (different type) too. It is usually related the video driver.
As a woraround, I used xbacklight (see How to change LCD brightness from command line (or via script)?) to change the display brightness, and put a call to a script with that command to lighdm startup code (/etc/lightdm/lightdm.conf, either display-setup-script, session-setup-script, or both).