cannot control brigtness of ubuntu 12.04.2 login screen

when i edited the following i can control the brightness after login in to the but i cannot control brightness of log in screen
also each time after booting brightness becomes full ie: it previously adjusted brightness changes

gksudo gedit /etc/default/grub

Change the line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to:
GRUB_CMDLINE_LINUX="GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"

Save the new file, close gedit and run:
sudo update-grub

Reboot to test

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gnome-control-center 1:3.4.2-0ubuntu0.11
ProcVersionSignature: Ubuntu 3.5.0-26.42~precise1-generic 3.5.7.6
Uname: Linux 3.5.0-26-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.1
Architecture: amd64
Date: Sun May 19 13:29:21 2013
ExecutablePath: /usr/bin/gnome-control-center
InstallationMedia: Ubuntu 12.04.2 LTS "Precise Pangolin" - Release amd64 (20130213)
MarkForUpload: True
ProcEnviron:
 SHELL=/bin/bash
 PATH=(custom, no user)
 LANG=en_IN
 LANGUAGE=en_IN:en
SourcePackage: gnome-control-center
UpgradeStatus: No upgrade log present (probably fresh install)
usr_lib_gnome-control-center:
 activity-log-manager-control-center 0.9.4-0ubuntu3.2
 deja-dup 22.0-0ubuntu3
 gnome-bluetooth 3.2.2-0ubuntu5
 indicator-datetime 0.3.94-0ubuntu2