Mark IV microdot camera

Microdots are text or photographic images that are reduced in size to prevent their viewing by unintended recipients. A microdot is usually shrunk down to the size and shape of a period or the dot of an i or j, and was usually sent by writing an innocent-looking letter containing periods, i's, or j's. This technique was invented by the Germans during World War II and then later used by many countries to pass messages through insecure postal channels. Later microdot technique used film with aniline dye, rather than silver halide layers and were ever harder for counter-espionage postal inspectors to find. Professor Zapp in Germany is claimed to have been the inventor of the technique. A WWII spy kit for microdot production was sometimes called a Zapp outfit.

After the erection of the Berlin Wall, 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 the microdot, 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 Plum duff , also known as spotted dick.

See also:

• Steganography
• A Scanner Darkly
• V-mail
• List of cryptography topics
• Microprinting

References

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