What is a Watermark?

When most of us think of a watermark, the first thing that comes to mind is money. If you hold a newer dollar bill up in the sunlight, you'll see a hidden image that proves the currency is real. These images are almost impossible to forge. In general, a watermark is any symbol or design that's hidden in a document, photo, or even in digital audio. They can be visible or totally invisible, they prevent counterfeiting, and they declare the original owner of a creative work. 

How Does a Watermark Work?

Typically, a watermark is hidden in a physical document by manipulating the thickness and density of the paper, in order to create a detailed design. This clever technique can be used to date a particular document, find out where it was produced, and much more. When light is shone through the paper, the thinner sections let more light pass through, while the thicker sections block it. This can be achieved with two main processes: dandy roll, or a complex cylinder mould. 

  • Dandy Roll: First introduced in 1826, the dandy roll is sort of like a paint roller mixed with a stamp. Thin wires on the roll, which have a similar consistency to a metal window screen, make an impression on the paper fibers as it is fed through.  
  • Cylinder Mould: This mould is similar to a dandy roll, but instead of wire mesh, it uses tonal depth to create a more distinct image. This technique was first used in 1848, and it allows for cleaner watermarks. Today, it is still a popular mould for watermarking currency and important documents.   

Digital Watermarks

Besides placing text over an image or document to help prevent theft, there are also digital watermarks that use secret data to include copyright information. With this technique, a data pattern is included in a digital photo, audio file, or video clip, which links the file to its original creator. These are designed to be hidden and fully traceable. 

For example, a record label may share early copies of a new album with journalists and PR people, but it will include a digital watermark to discourage piracy. If someone ends up leaking the music, it's easy to trace those files back to a particular email account or personal profile, and have proof of their illegal behavior. Only the watermark's creator can use special software to decipher the embedded data and learn who has shared it. For audio, perhaps the most popular watermarking method is spread spectrum audio watermarking (SSW), which spreads a narrow-band signal over a huge frequency range so that it's imperceptible. 

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