I think a way to do this in software if you cannot change the brightness in hardware is through xrandr --output LVDS1 --brightness on the command line This will in software make it appear to be more dim or less bright. I found this info on http://manpages.ubuntu.com/manpages/trusty/man1/xrandr.1.html
This is just making the screen appear more or less bright for each pixel however and will not improve battery life by reducing brightness in this way. As the backlight is still on the same strength as a given light takes more power to shine brighter.
So only do this if xbacklight doesn't work. To get a similar brightness to xbacklight -set 50 you would run xrandr --output LVDS --brightness 0.5. Of course if it is a desktop monitor it will be either VGA1 DVI1, or HDMI1 according to the input.
If it is an external monitor you can also change the brightness with buttons on the monitor even if you do not have the backlight property on your monitor like I don't have on my asus vs229-hp.