i3 brightness

if brightness settings work in xfce (xfce4-power-manager in fedora20) but not in i3, then you may want to use its /usr/sbin/xfpm-power-backlight-helper:

bin/brightness:

#! /bin/sh
if test "x`basename $0`" = 'xbrightness'; then
echo "don't use me directly"
exit 1
fi
CUR=`pkexec /usr/sbin/xfpm-power-backlight-helper --get-brightness`
MAX=`pkexec /usr/sbin/xfpm-power-backlight-helper --get-max-brightness`
STEP=`expr $MAX / 20`

bin/brightness+:

#! /bin/sh
DIRNAME=`dirname $0`
. "$DIRNAME/brightness"
NEW=`expr $CUR + $STEP`
if test $NEW -gt $MAX; then
        NEW=$MAX
fi
pkexec /usr/sbin/xfpm-power-backlight-helper --set-brightness $NEW

bin/brightness-:

#! /bin/sh
DIRNAME=`dirname $0`
. "$DIRNAME/brightness"
NEW=`expr $CUR - $STEP`
if test $NEW -lt 0; then
        NEW=0
fi
pkexec /usr/sbin/xfpm-power-backlight-helper --set-brightness $NEW

assigning these scripts to i3 keys is documented everywhere: