A microdot is text or an image substantially reduced in size onto a small disc to prevent detection by unintended recipients. Microdots are normally circular around one millimetre in diameter but can be made into different shapes and sizes and made from various materials such as polyester and also metal. The name comes from the fact that the microdots have often been about the size and shape of a typographical dot, such as a period or the tittle of a lowercase i or j. Microdots are, fundamentally, a steganographic approach to message protection.
In 1870 during the Franco-Prussian War, Paris was under siege and messages were sent by carrier pigeon. Parisian photographer René Dagron used a photographic shrinking technique to permit each pigeon to carry a high volume of messages, as pigeons have a quite restricted payload capacity. However, the images were not as small as modern microdots.
An actual microdot technique was used for steganographic purposes in Germany between World War I and World War II. 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. Professor Zapp from Germany is claimed to have been the inventor of the technique, and a World War II spy kit for microdot production was sometimes called a Zapp outfit. However, Emanuel Goldberg is also said to have invented the modern technique. Like much in the history of espionage and subversion, there is controversy.
In Germany after the Berlin Wall was erected, special cameras were used to generate microdots which were then adhered to letters and sent via normal means. These microdots often went unnoticed by inspectors, and information could be read by the intended recipient using a microscope.
British mail censors sometimes referred to microdots as "duff" since they were distributed here and there throughout letters rather like raisins in the British steamed suet pudding called "plum duff".
Modern usage 
Microdot Identification 
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Microdot identification is a process where tiny identification tags are etched or coded with a given vehicle VIN, asset identification number or a unique serial number. Unique personal identification numbers (PIN), asset identification number or customized customer data entries are also available. The microdots are brushed or sprayed onto the key parts of an asset to provide complete parts marking. The technology was developed in the United States in the 1990s before being commercialized by various manufacturers and distributors around the world.
Popular culture 
- In the motion picture Mission Impossible 3 a microdot was hidden on the back of a postage stamp and contained a magnetically stored video file.
- In Superman #655 (Vol. 1, Sep. 2006), Clark Kent uses various microdots implanted throughout a suspense novel to read not only the novel but also numerous other works on various topics. The microdots were used here to further explore Superman's newly-enhanced mental capabilities.
- In You Only Live Twice, Tiger tells James Bond that his men found a microdot on a captured SPECTRE photograph, which he enlarges for Bond.
- In the 1966 movie Arabesque a microdot was hidden in the eye of a goose on a parchment of hieroglyphs.
- One of Philip K. Dick's characters in A Scanner Darkly tells a drug-induced story wherein a worker at the local microdot factory had tracked the company's entire inventory out into the parking lot on the sole of his shoe.
- In the Nancy Drew PC game, Phantom of Venice, a clue is hidden using a microdot on an exclamation point.
- The 2003 film Paycheck uses a very realistic rendering of a microdot as a key plot element. The handling of microdot technology in the film is worth noting as the viewer is shown both how well a microdot can be made to blend into a complementary environment as well as how much information such a dot can carry.
- In the White Collar episode "As You Were" a microdot was used to send a covert message to Special Agent Clinton Jones.
- In the Covert Affairs episode "Sad Professor" a microdot was used by one of the characters to store intelligence related to an operation that a language professor used who previously worked for the CIA.
- In the The Venture Bros. episode Powerless In the Face of Death; while in prison, the character Tiny Joseph comments that "they don't usually write microdots by hand."
- In the television series Get Smart (Season 1, Episode 21 - "Dear Diary") Agent 86 and Agent 99 are shown the first "microdot" in the Spy City Museum. The comedic value is in the microdot being the size of a small plate.
- Kipper, Gregory. Investigator's Guide to Steganography. Boca Raton: Auerbach Publications, 2003.
- Hayhurst, J.D. (1970). The Pigeon Post into Paris 1870-1871. (privately published).
- Buckland, Michael (2006). Emanuel Goldberg and His Knowledge Machine: Information, Invention, and Political Forces. New Directions in Information Management. Libraries Unlimited.
- Histories, Heritages, and the Past: The Case of Emanuel Goldberg, Michael K. Buckland[dead link]
- Wonko'S Web Pages
White, William. The Microdot: History and Application. Williamstown, NJ: Phillips Publications, 1992.