This script prevents lock screen from dimming. Tested on Ubuntu 16.04.2 LTS with default Unity interface.

install_prevent_lock_screen_dim.sh

#!/bin/bash
# Copyright (c) 2017 @yuan3y
# Released under MIT License
# Tested on Ubuntu 16.04.2 LTS with default Unity interface
#
# USAGE:
#   chmod +x install_prevent_lock_screen_dim.sh
#   ./install_prevent_lock_screen_dim.sh

scriptFile=~/.prevent_lock_screen_dim.sh
desktopFile=~/.config/autostart/.prevent_lock_screen_dim.sh.desktop

if hash xdotool 2>/dev/null; then
    echo "Checking xdotool exists: PASS"
else
    echo "Checking xdotool exists: FAILED"
    while true; do
        read -p "Do you wish to install xdotool the commandline X11 automation tool?" yn
        case $yn in
            [Yy]* ) sudo apt-get install xdotool; break;;
            [Nn]* ) exit;;
            * ) echo "Please answer yes or no.";;
        esac
    done
fi

cat > ${scriptFile} <<_EOF
#!/bin/bash
# This script prevents lock screen from dimming while it is running.
# 
# Copyright (c) 2017 @yuan3y
# Released under MIT License
# Tested on Ubuntu 16.04.2 LTS with default Unity interface
#
# To uninstall the script, 
# # 1) Kill all running instance,
# kill \$(ps aux | grep '[p]revent_lock_screen_dim.sh' | awk '{print \$2}')
# # 2) remove it from Startup Applications, or run the following command
# rm ${desktopFile}
# rm ${scriptFile}
dbus-monitor --session "type='signal',interface='com.ubuntu.Upstart0_6'" |
while read x; do
    case "\$x" in 
        *"started"*) sleep 1; xdotool key "Escape";;
    esac
done
_EOF

chmod +x ${scriptFile}

cat > ${desktopFile} <<_EOF
[Desktop Entry]
Type=Application
Exec=${HOME}/.prevent_lock_screen_dim.sh
Hidden=false
NoDisplay=false
X-GNOME-Autostart-enabled=true
Name=Prevent Lock Screen Dimming
Comment=Preventing Lock Screen from Dimming
_EOF

${scriptFile} &>/dev/null &
echo "Installation is done. Your lock screen will be prevented from dimming."