Same thing happened to me. In my case xbacklight had a problem with choosing
the right device in /sys/class/backlight/
To fix this I found a solution on
https://wiki.archlinux.org/index.php?title=Backlight&oldid=310785#Backlight_fails_to_adjust_on_Intel_chipsets_with_Kernel_3.13
1. Create /etc/X11/xorg.conf.d/20-intel.conf
2. Put the following lines of code in there
Section "Device"
Identifier "Backlight fix"
Driver "intel"
Option "Backlight" "intel_backlight"
EndSection
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https://bugs.launchpad.net/bugs/989603
Title:
xbacklight does not work in Lubuntu 12.04
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