A miniaturized communication system involving the reduction of a photograph to a scale where it can be read only with the aid of a microscope and is therefore easy to conceal. Microphotography was developed in Germany in 1938 and was adopted with enthusiasm by the Abwehr as an alternative secret writing, which was susceptible to detection by random testing. This ingenious solution, patented in 1938 by Zeiss, solved the problem of carrying potentially incriminating documents and messages. Photographing the item and then reducing it in size by a ratio of 200:1 to a microdot required specialist knowledge, training, and equipment but was a major breakthrough in concealed writing, which hitherto had depended on secret inks that could be detected under certain lighting conditions or chemical treatment. The use of microdots was first revealed in the United States by an Abwehr spy, William Sebold, in 1941. Suitably impressed, the Federal Bureau of Investigation hired an optical expert, James E. Dunlop, from the medical laboratory at Johns Hopkins Hospital, to study photo reduction techniques.
Code-named DUFF by MI5 in 1940, microdots continue to be a practical method of conveying large quantities of information without the need for sophisticated equipment.