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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".
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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.
White, William. The Microdot: History and Application. Williamstown, NJ: Phillips Publications, 1992.