The detection process makes use of the optical phenomenon known as laser speckle. Light coming from a focused laser is coherent, or in phase, but when it strikes a microscopically rough surface like a piece of paper, the light is scattered, producing a pattern of light and dark "speckles." The scanner's photodetectors digitize and record this pattern.
According to Cowburn's research, as published July 28 in the journal Nature, the unique speckle pattern of a sheet of paper remains recognizable even after crunching the paper into a ball, soaking it in water, baking it at 180 degrees Celsius (350 degrees Fahrenheit) for 30 minutes, scrubbing it with an abrasive cleaning pad or scribbling over it with a big black marker.
A cross-correlation algorithm that assesses the degree of similarity between the base-line scan and the new scan allows the paper's identity to be verified. The odds of two pieces of paper having similar patterns are greater than 10 to the 100th power to one.
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