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Surfaces have "fingerprints"

Russell Cowburn and co-workers at Imperial College London, Durham University and the University of Sheffield used a phenomenon called "laser speckle" to examine the structure of different surfaces. This technique is already routinely used to measure surface roughness in metal and paper and for visualizing blood in vivo. They scanned a focused laser beam over a sheet of white paper and used photodetectors to record the intensity of the light reflected from four different angles.

The physicists then quantified how much random fluctuations on the paper differed from the mean value (called the zero positional shift) and converted these values into 1s and 0s to obtain the fingerprint code. They obtained different codes for different sheets from the same pack (see figure), and achieved similar results for plastic credit and identity cards and cardboard packaging. [b]Moreover, a sheet of paper could be identified even after it had been screwed up into a ball, submerged in water, baked at 180