It’s not just a spy movie cliche: secret messages often appear as microdots. (The Thought Emporium) researched the history of microdots and even made some of them, which – to quote the video below – turned out to be “easier than you might think, but also more difficult in other ways.”

When you want to hide a secret message, you face two problems. The first is that the message must be encrypted so that only the recipient can read it. But in many cases, you also want the existence of the message to remain secret. If an enemy spy sees you with a folder full of encrypted documents, your cover is blown, even if he doesn’t know what the documents say.

Today, steganography techniques allow you to hide messages in innocent-looking images or data files. For many years, however, microdots were the gold standard for hiding secret messages and surreptitious photos. The microdots are usually no larger than a millimeter, making them easy to hide in plain sight.

The idea behind the microdots is simple. Basically, they are tiny pieces of film that can only be read under magnification. After all, you can shrink a beach photo down to a relatively small negative, so why not a document?

The example microdots use ISO 50 film to ensure a fine grain pattern, although manufactured microfilm might have been a better choice for this task. Apparently real spies used special films that use aniline dyes to avoid film grain issues.

However you do it, you need a way to take high-resolution images, put them on film, and then crop the film so it can be hidden. While microdots were put on pigeons as early as 1870, it wasn’t until 1925 that it was technically possible for microdots to represent a page as small as ten square micrometers. This was a two-step process, so these homemade microdots won’t be as dense between the film and the one-step processing.

If all this is too much trouble for you, you can always resort to invisible ink. Or you can use a more modern technique.