A microdot is text or an image substantially reduced in size to prevent detection by unintended recipients. Microdots are normally circular and around one millimetre 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 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.
Improvement in technology since then has made even more miniaturization possible.
At the International Congress of Photography in Paris in 1925 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 a surface of 0.01 sq. mm. 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 steganographic purposes was first used 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 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". However, there never was a Professor Zapp at that university and microdot historian William White has denounced Hoover's article as a "concoction of semitruths and overt disinformation".
Nevertheless, this article was reprinted, translated, and widely and uncritically cited in the literature on espionage. 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 the Second World War, was in Dresden and taught spies how to make microdots. A World War II spy kit for microdot production was sometimes called a Zapp outfit.
Mark IV microdot camera
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. 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".
Microdot identification is a process where tiny identification tags are etched or coded with a given number, or for use on vehicles, a 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.
In South Africa it is a legal requirement to have microdot fitted to all new vehicles sold since September 2012 and to all vehicles that require police clearance.
Most printers print in addition to the documents requested on the pages tiny yellow dots containing printer serial number and time stamp. These are not microdots, but arrays of difficult-to-see dots across the printed page in an encoded pattern.
In the 2006 motion picture Mission: Impossible III 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 the 1967 movie 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.
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 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 C.I.D. Episodes 201 and 202, "Case of the Multiple Puzzles", a microdot was sold by an Intelligence Bureau officer to terrorists. The microdot had information about missile technology of India.
Lee Harvey Oswald famously wrote "micro dots" in his address book underneath the address for a printing company he worked for in 1962 and 1963.
In the 1965 television series Get Smart (Season 1, Episode 21 - "Dear Diary" original air-date Feb 12, 1966) 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.
In the 1968 television series It Takes a Thief (Season 1, Episode 8, "A Spot Of Trouble"), Agent Mundy is called in when sensitive plans for a weapon are stolen and later learned have been converted to a microdot.
In the Blake & MortimerS.O.S. Meteors comic book, the foreign organization responsible for altering the weather patterns over Europe uses microdots embedded in letter envelopes to relay projected weather data to their clandestine stations. The microdot is however misattributed to "famous German inventor" Zapp.
In the television series The Avengers, the 1961 episode "One for the Mortuary"  has microdots and their transport as a major plot theme.
In Margaret Atwood's 2019 novel "The Testaments," the sequel to "The Handmaid's Tale," microdots are used for communication between a high official of The Republic of Gilead and Mayday resistance members in Canada. The microdots were smuggled back and forth in print brochures and also inserted into the tattoo of a defector from Gilead to Canada.
^Kipper, Gregory. Investigator's Guide to Steganography. Boca Raton: Auerbach Publications, 2003.