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A microdot is text or an image substantially reduced in size to prevent detection by unintended recipients. Microdots are normally circular and around in diameter but can be made into different shapes and sizes and made from various materials such as polyester or metal. The name comes from microdots often having been about the size and shape of a typography dot, such as a full stop or the tittle of a lowercase i or j. Microdots are, fundamentally, a steganography approach to message protection.
Improvement in technology since then has made even smaller miniaturization possible.
At the 1925 International Congress of Photography in Paris, Emanuel Goldberg presented a method of producing extreme reduction microdots using a two-stage process. First, an initial reduced negative was made, then the image of the negative was projected from the eyepiece of a modified microscope onto a collodium emulsion where the microscope specimen slide would be. The reduction was such that a page of text would be legibly reproduced in an area of 0.01 mm2. This density is comparable to the entire text of the Bible fifty times over in one square inch. Goldberg's "Mikrat" (microdot) was prominently reported at the time in English, French and German publications.
A technique comparable to modern microdots for steganography purposes was first used in Germany during the Interwar period. It was also later used by many countries to pass messages through insecure postal channels. 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". This article was reprinted, translated, and widely and uncritically cited in the literature on espionage. There never was a Professor Zapp at that university; (1992). 9780932572196, Phillips Publications. ISBN 9780932572196 Hoover's Zapp has been wrongly identified with Walter Zapp, inventor of the Minox camera, which was used by spies but did not make microdots. Hoover appears to have conflated Emanuel Goldberg, who was a professor in Dresden, with Kurt Zapp who, late in World War II, was in Dresden and taught spies how to make microdots.
Some printers print, in addition to the document contents requested, tiny yellow dots containing the printer serial number and a time stamp.
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