 Especially during the war time, UV-invisible was widely used in militaries to hide secret messages and important addresses to avoid their enemy to have detail information on their next step of action as this will put them on advantage by providing them time to prepare and giving them chances to destruct their plans.During World War II., a battle between laboratories to develop the best UV invisible ink was revealed between the Allies & the Axis powers.They were desperately trying to develop new, better UV-invisible inks that can only be revealed by a very few type of UV light with a very particular size of wavelength.