The microdot is a page-sized photograph that has been reduced to 1 mm in diameter. The microdot became a popular and commonly used form of steganography during World War II. The process of creating a microdot is straightforward, but requires a few specialized pieces of equipment. First, a photograph of the message is taken, and this reduces it to roughly the size of a postage stamp. Next, the image is shrunk further with a reverse microscope, bringing it down to 1 millimeter. The negative is then developed and the image is punched out of the film. A common way to do this was with a syringe needle in which the point had been filed down. Once the needle separated the dot from the rest of the film, it was placed on the cover text, over a period or under a stamp, and cemented in place.
Professor Walter Zapp is credited with creating a device that could perform most of these processes mechanically.