The printing service provider Schaetzl and the Slovenian company Hooray Studios are adding a new dimension to personalized children's books: in addition to the normal story, their book “Glow in the Dark” offers young readers another hidden story which only lights up in the dark. For this, the new HP Indigo ElectroInk Invisible Yellow is used. The additional story within the book is only visible with a special UV lamp, providing double the reading fun.