Using technology akin to X-ray vision, scientists can read closed books, identifying letters printed on stacks of paper up to nine sheets thick.
This finding could lead to office machines that can scan reams of paper at once, or help researchers scan ancient books that are too fragile to open. The researchers also said it could perhaps help spies read mail without opening envelopes.
The prototype device uses terahertz radiation, the band of electromagnetic radiation between microwaves and infrared light. Previous research has found that terahertz rays, or T-rays, possess a number of advantages over X-rays, ultrasound waves and other kinds of radiation that can penetrate surfaces. For instance, terahertz rays can distinguish between ink and blank paper in a way that X-rays cannot. They can also scan across depths to yield higher-resolution images than ultrasound can accomplish, according to the researchers.
The new system relies on how different chemicals absorb different frequencies of terahertz radiation to varying degrees, the scientists said. As such, it can tell the difference between paper that has ink on it versus paper that does not.

Moreover, the new system exploits the fact that air and paper each bend light to a different degree, and that pages of a book trap air pockets between them. These pockets may only be about 20 microns deep — about one-fifth of the average width of a human hair — but this can be enough for the device to distinguish the signals from different pages of a book , the researchers said.

The researchers used a terahertz camera to scan a stack of card-size, 300-micron-thick sheets of paper. Each had a single letter about 0.3 inches (8 millimeters) wide written on only one side in pencil or ink.
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