Answer: Pantograph

Many documents contain a hidden message that only appears when photocopied. This message, found on checks, birth certificates, prescription slips, and other documents prone to fraudulent reproduction is known in the printing industry as a “void pantograph”. It is named such as the most commonly used word is “VOID” and the term pantograph is a Greek term that means copy writing.

While the methods by which void pantographs are created vary, some printers use off-set patterns of tiny perpendicular lines while others use tiny dots; the mechanism of action relies on the limitations of the photocopying process. While photocopiers do a pretty great job of making copies of simple text and even photographs, they function as a type of low-pass filter for certain patterns. When you make a copy of the security-marked document, the invisible-to-the-human-eye words appear clearly in the copy.