Scientists have developed an innovative light-based
technique to create secure, invisible watermarks that can be used to prevent
photos, videos and books from being illegally copied and distributed. “In our
research, we use a complex pattern of light, or diffraction pattern, as a
unique watermark,” said Yishi Shi, from the University of Chinese Academy of
Sciences, China. “The invisible watermark is embedded into the content we are
trying to protect. Imperceptibility is one of the most significant advantages
of optical watermarking,” said Shi. The new approach encodes the optical
watermark in a single step. It is faster and uses a less complex optical setup
than other optical watermarking approaches previously pursued, researchers
said. The technique can also be used to optically encrypt data or to hide
information within images. The method is based on a technique called
single-shot ptychography encoding (SPE) that 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. SPE is also less prone to error
than other methods and uses a simpler optical setup. In addition to conducting
numerical simulations to test their method, the researchers carried out an
optical experiment showing the usefulness of SPE. “Most methods for optical
watermarking have only been demonstrated with simulations. Our experiment shows
that our method is suitable for practical optical watermarking,” said Shi. For
the optical experiment, the researchers used SPE to create a complex watermark
consisting of a diffraction pattern of multiple tiny spots. Prior to embedding
the watermark into a host image, they used 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. The spot size can be
reduced to smaller than 10 microns, which helps prevent degradation of the host
image. Once a watermark is embedded into digital media, there are multiple ways
to detect it to check for authenticity. If someone knows an optical watermark
is present, it can be detected by subtracting the host image from the
watermarked image and then using a special security key and extraction
algorithm. For cases where the presence of a watermark is unknown, the
watermark could be extracted using existing algorithm-based detection methods.
The research was published in the journal Optics Express. -PTI
Source: DNA-26th-November;2016
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