dim all your connected shiny bright displays via bash comfortably after you checked that xbacklight really doesn't work on your hardware: use "bright" :-)

bright

#!/usr/bin/env bash
# sets the brightness via xrandr on all connected displays
# requirement: {sudo apt-get update && sudo apt-get install x11-xserver-utils zenity}
# installation: copy "bright" to ~/bin/ and {chmod +x ~/bin/bright} it
# usage: "bright" without args shows a zenity scroll dialog using this script
# author: https://github.com/hbartnick 2015

function set_brightness {
  for output in ${outputs[@]}; do
    xrandr --output $output --brightness $1
  done
}

function cancel_hook {
    "$@"
    local status=$?
    if [ $status -ne 0 ]; then
       set_brightness $cur
    fi
    return $status
}

outputs=($(xrandr --verbose|grep -oP '^.*(?= connected)'))
if [[ -t 0 ]]; then
  if [[ "$1" == "-n" ]]; then
    xrandr --verbose | grep -m 1 -i brightness | cut -f2 -d ' '
    exit 0
  fi
  if [[ -z $1 ]]; then
    cur=$(command $BASH_SOURCE -n)
    cancel_hook zenity --scale --title="brightness" --text="" --value=$(echo "(${cur}*100)/1"|bc) --print-partial | {
      while read x; do
        echo "scale=2;${x}/100" | bc | command $BASH_SOURCE;
      done
    }
  else
    set_brightness $1
  fi
else
  while read f ; do
    set_brightness $f
  done
fi