Microdot

A microdot is text or an image substantially reduced in size to prevent detection by unintended recipients.

[2] At the 1925 International Congress of Photography in Paris, Emanuel Goldberg presented a method of producing extreme reduction microdots using a two-stage process.

Later microdot techniques used film with aniline dye, rather than silver halide layers, as this was even harder for counter-espionage agents to find.

A popular article on espionage by J. Edgar Hoover in the Reader's Digest in 1946 attributed invention of microdots to "the famous Professor Zapp at the Technical University Dresden".

[citation needed] In Germany after the Berlin Wall was erected, special cameras were used to generate microdots which were then attached to letters and sent through the regular mail.