Solved Lenovo G505s Brightness Control

Dear community,
some days ago I have fixed a broken laptop by repairing the DC socket and changing the HDD. Now FreeBSD-11.1 Release with x11-wm/xfce4 has been installed. The machine uses the radeon driver. Unfortunately tweaking with sysctl(8) and acpi_video(4) to adjust the backlight did not work. For my setup it required two steps to adjust the backlight. First find out the monitor by
xrandr --listmonitors
which returned in my case
0: +eDP 1280/344x720/194+0+0 eDP
With eDP as the monitor I could use x11/xrandr to set the brightness, for example to 80% by
xrandr --output eDP --brightness 0.8
This line plus the usual shebang and the path to the executable of x11/xrandr is placed in a script which is called at startup of x11-wm/xfce4. At least my wife is happy with the permanent change of 100% brightness to 80%. The lines related to x11/xrandr must be run as the user because root as no access to the X Window system. May be this method is useful for others as well.