A new light-based technique developed at the University of Chinese Academy of Sciences could make it more practical to create secure, invisible watermarks that can be used to detect and prosecute counterfeiting.

“In our research, we use a complex pattern of light, or diffraction pattern, as a unique watermark,” says Yishi Shi, of the university's College of Materials Science and Opto-Electronic Technology. “The invisible watermark is embedded into the content we are trying to protect. Imperceptibility is one of the most significant advantages of optical watermarking.”

Compact discs, books and videos are some of the goods that could be protected against counterfeiting with the invisible watermark. Image credit: Pixabay.The new method is based on a technique called single-shot ptychography encoding (SPE), which uses multiple partially overlapping beams of light to generate a diffraction pattern from a complex object. Unlike other methods, SPE can encode the optical watermark in a single exposure with no mechanical scanning. It is also less prone to error than other methods and uses a simpler optical setup, according to the researchers.

In addition to conducting numerical simulations to test their method, Shi's research team carried out an optical experiment to demonstrate the utility of SPE. Using SPE they created a complex watermark consisting of a diffraction pattern of multiple tiny spots. Prior to embedding the watermark into a host image, they applied computer processing to remove any repeated data and to scramble the diffraction pattern, making it easier to embed the watermark and further improving its security. They were able to reduce the spot size to smaller than 10 microns, which helps prevent degradation of the host image.

The researchers are now working to apply SPE to dynamic watermarking, which creates watermarks from objects that change quickly. For example, the variations that occur within a biological cell could be recorded and used to create a watermark. They also intend to refine and enhance the technology's imaging quality and use it for multi-image, and even 3D, watermarking.