A microdot is text or an image substantially reduced in size onto a 1mm 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. 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. It is, 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. A Parisian photographer named 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. A Professor Zapp in Germany is claimed to have been the inventor of the technique, and a WWII spy kit for microdot production was sometimes called a Zapp outfit. However, Emanuel Goldberg is also alleged to have been the inventor of the modern technique , under which hypothesis Professor Zapp's connection would be a fiction. Like much in the history of espionage and subversion, there is controversy.
After the Berlin Wall was put up, special cameras were used to generate microdots which were then adhered to letters and sent via normal means. Owing to the extremely small size of microdots, these messages often went unnoticed by inspectors and information could then 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 spotted dick (or "plum duff").
World-wide, various governments, companies, and manufacturers have begun using microdot identification to protect their assets. World-wide the takeup of this product is less than 1% but growing in certain segments based upon the ease and availability of the technology. The microdots are a covert technology but can be read with a 60x magnifying scope. This covert parts-marking technology is aimed at detering thieves, particularly car thieves, who would have otherwise been able to rebirth vehicles as well as sell stolen vehicle parts as legitimate ones. Theft of vehicle parts in the US alone now exceeds $10bn.
The following manufacturers use microdots in some of their vehicles, in some territories.
White, William. The Microdot: History and Application. Williamstown, NJ: Phillips Publications, 1992.