By default the xbacklight command does not work on Intel HD Graphics:
Workaround:
sudo nano /usr/share/X11/xorg.conf.d/10-intel.conf:
insert:
Section "Device" Identifier "Card0" Driver "intel" Option "Backlight" "/sys/class/backlight/intel_backlight" EndSection
now add to startup / autostart (lxde) :
xbacklight -set 30