Here is a patch you can do.
Create this script with the name .modificarBrillo.sh (in my case I created it in my home folder: ~/.modificarBrillo.sh)
#!/bin/bash
if [ -z "$1" ]; then
echo "ERROR: Tiene que introducir un parámetro: \"a\" para aumentar o \"d\" para disminuir"
exit
else
if [ "$1" != "a" ] && [ "$1" != "d" ]; then
echo "ERROR: el parámetro de entrada sólo puede ser o \"a\" para aumentar el brillo o \"d\" para disminuirlo"
exit
fi
fi
MAX_BRILLO=`cat /sys/class/backlight/intel_backlight/max_brightness`
MIN_BRILLO_ABSOLUTO="0"
MIN_BRILLO="100" #el brillo mínimo puede ser 0 pero eso deja la pantalla completamente a oscuras
INTERVALO=`expr $MAX_BRILLO - $MIN_BRILLO_ABSOLUTO`
INTERVALO=`expr $INTERVALO / 10`
brillo=`cat /sys/class/backlight/intel_backlight/actual_brightness`
if [ "$1" = "a" ]; then
let "brillo = brillo + INTERVALO"
if [ "$brillo" -gt "$MAX_BRILLO" ]; then
let "brillo = MAX_BRILLO"
fi
else
let "brillo = brillo - INTERVALO"
if [ "$brillo" -lt "$MIN_BRILLO" ]; then
let "brillo = MIN_BRILLO"
fi
fi
echo "$brillo" | tee /sys/class/backlight/intel_backlight/brightness
However as the previous script needs execution permission and /sys/class/backlight/intel_backlight/brightness can only be edited by root and you have to execute in terminal:
chmod a+x ~/.modificarBrillo.sh
sudo chmod a+w /sys/class/backlight/intel_backlight/brightness`
The last command has to be executed every startup because the permissions of the brightness file are renewed with the startup. For doing so sudo vim /etc/rc.local
and add the command sudo chmod a+w /sys/class/backlight/intel_backlight/brightness
before the "exit 0" line
Finally you should install xbindkeys to assgin the Function key to execute the script.
In my case I add the lines to the configuration file ~/.xbindkeysrc
#Aumentar brillo
"/home/alvaro/.modificarBrillo.sh a"
m:0x0 + c:233
XF86MonBrightnessUp
#Disminuir brillo
"/home/alvaro/.modificarBrillo.sh d"
XF86MonBrightnessDown
But you could also install the program xbindkeys-config to do the proccess graphically.