BackLight not working on your laptop? try this!

Every since I got this new HP laptop about a year ago,
I was unable to change the backlight brightness, this caused many issues,
most noticeably with battery life. and after a year and 3 months I finally found the Fix:

Special thanks to Chris Moore at askubuntu.com

All you have to do is edit /etc/default/grub
and find the line : GRUB_CMDLINE_LINUX_DEFAULT="
and add acpi_backlight=vendor to the end of it.
so the full line should read:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

Save the file and then run sudo update-grub
after you reboot, you will now be able to change your brightness! Minor issue: sometime after my screen has been locked for a while, the brightness dose not turn on but the password screen comes up, after you type your password and press enter, you will be able to turn your brightness back up (using keyboard). weird, I'll let you guys know if I find a fix. source: http://askubuntu.com/questions/21523/screen-brightness-not-changing-on-acer-5742-notebook/148242#148242