Unlike previous replies, this is a useful one that will solve the issue .
The solution to this bug.
Until this bug is fixed, here is how to fix the backlight issue that works for most users.
First of all, one problem some have is when they boot the OS after installed there's no backlight, and sometimes there's no backlight in the installer either. To get the backlight to come on so you can install Ubuntu, do this:
1. Insert your Ubuntu install disc (or usb drive) and reboot.
2. When you boot the disc or usb drive and you see the "Purple Ubuntu screen" or Ubuntu splash, right away, hold the Fn button and dim your screen brightness then brighten it back up. If you forget to do this and the splash (or purple screen) is gone then the backlight probably wont work and you can't install Ubuntu.
Once ubuntu is installed and you boot from the HDD, do the same when you boot Ubuntu. When you see the purple screen, right away Hold Fn button and dim the screen down and up. Most emachines, being made by acer, to do this, hold Fn button and then hit the left arrow key then the right arrow key.
By doing this, Ubuntu will boot with the backlight. I don't know why, but it does.
Now, that's a bit annoying to fix, and once Ubuntu is booted, you can't adjust your backlight. What we want to do is make ubuntu turn the backlight on automatically when you boot it and also, allow you to adjust your backlight once Ubuntu is booted. To do this:
and hit enter.
gksudo gedit /etc/rc.local
Above exit 0, put in:
Save the file, and exit Gedit. If you get a popup message the "untitled document" isn't save, ignore it and close without saving.
setpci -s 00:02.0 F4.B=00
This fix will automatically make the backlight turn on upon boot.
Now to ensure you can adjust your backlight once Ubuntu has booted:
In terminal, enter the command:
In the file, find the line:
gksudo gedit /etc/default/grub
Change that line to:
Save the file. Exit gedit. If prompted to save untitled document, don't, choose close without saving.
Now in terminal, type the command:
Wait for the process to finish then exit terminal. Reboot your PC.
Once rebooted, you will notice the purple screen, but you don't need to adjust your backlight this time. You'll notice once you pass the boot purple screen, the backlight will go out. Don't worry. Wait a few seconds, then the backlight will come on, and ubuntu will boot up and you'll hear the Ubuntu startup sound.
You will also notice that now you can also use the Fn key and use the right and arrow keys to adjust the brightness. (Or whatever keys you use to adjust the brightness). YAY! It's fixed.
A couple of things to consider:
For some users, the backlight still won't automatically turn on. You can however use the brightness controls after Ubuntu is booted to turn the backlight on. If the backlight after doing these fixes still wont turn on, go back to this command and run it "gksudo gedit /etc/rc.local", find the line "setpci -s 00:02.0 F4.B=00" and change it to "setpci -s 00:02.0 F4.B=FF". (without quotes of course). Save and reboot.
Also, you'll notice if you go into standby (and I think Hibernate as well), the backlight will probably be off. I'm not aware of a way to fix this but to get the backlight back on use the Fn key and adjust your backlight to get it back on.
For some users, you may also notice that the backlight adjust keys are backwards. Mine aren't, but some users' are. I am not aware of a way to fix this but it's not that big of a bug. So adjust your backlight use the dim button to brighten the screen and the brighten button to dim the screen .
If you have any questions, I'll try to remember to keep an eye on this topic so I can do any further assistance.
It's a really annoying bug and a bug report is filed:
Hope this helps