Laptop users still have the backlight issue in Ubuntu 14.04. Every time Ubuntu boots up you get the maximum level of screen brightness.
Here’s a workaround by adding a startup script which will automatically adjust screen brightness when Ubuntu boots up.
To get started:
Preparation: Open your file browser and navigate to “Computer→ sys → class → backlight” directory. You’ll see two or three folders there.
In each folder there’s a file called actual_brightness, you can see its content (brightness value) through the thumbnail icon.
actual-brightness
Remember them and do:
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Change your screen brightness through Fn + Arrow key.
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Check out the actual_brightness files in each folder (Re-open each folder to refresh the value).
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Does the value change?
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Remember the folder name in which the value of actual_brightness changes.
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Replace acpi_video0 in below commands with that folder name
1. Press Ctrl+Alt+T on keyboard to open the terminal. When it opens, run the command below will give you the maximum level of your laptop backlight:
$ cat /sys/class/backlight/acpi_video0/max_brightness
Mine is 9, so I can set backlight level from 0 to 9.
2. Run the command below one by one. You’ll get the super user privilege and open the /etc/rc.local file, a script executed at the end of each multiuser runlevel, with gedit editor.
$ sudo -i
$ gedit /etc/rc.local
Add the line below before the last. Change the number 0 to the brightness level you want.
$ echo 0 > /sys/class/backlight/acpi_video0/brightness
That’s it. Enjoy!
(source: http://ubuntuhandbook.org/index.php/2014/04/save-screen-brightness-settings-in-ubuntu-14-04/)