When I boot up with 2 monitors, or when I connect the second monitor to my laptop, I can change brightness of the laptop screen in the usual sense.
After disconnecting the second monitor, when I change the brightness, the animation at the top right corner seems to be changing, but it doesn't effect the screen brightness.
the value in /sys/class/backlight/acpi_video0/brightness does change, so writing there is not the solution either.
Ubuntu 14.04 LTS, 3.13.0-30-generic #55-Ubuntu SMP Fri Jul 4 21:40:53 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Thanks in advance.