ESDA (electrostatic detection apparatus) is the best technology for detecting indented (invisible to the eye) writing on documents. ESDA works by creating an invisible electrostatic image of indented writing, which is then revealed by the application of charge-sensitive toners.

The sensitive imaging process reacts to sites of microscopic damage to fibers at the surface of a document, which have been created by abrasive interaction with overlying surfaces during the act of handwriting.