Using a type of radiation called terahertz radiation, researchers at MIT have devised an imaging method that can read closed books. With a stack of paper and a camera that emits short bursts of terahertz radiation, the team has been able to correctly identify letters printed on the paper to a depth of nine sheets.

Terahertz radiation is a type of electromagnetic radiation. It sits between microwave and infrared radiation, and is useful for non-destructive materials analysis, since it can easily distinguish between substances, as different chemicals absorb and reflect it differently. In this way, it can distinguish between ink and blank paper. The camera emits the radiation, and a built-in detector receives the reflections.

At the moment, it can only see to a depth of 20 pages, and accurately decipher letters to nine pages, because any farther than that and the signal to noise ratio drowns out the signal. But the team is working on improving it. It could be very useful for studying old and fragile books and manuscripts, for example.

"The Metropolitan Museum in New York showed a lot of interest in this, because they want to, for example, look into some antique books that they don't even want to touch," said corresponding author Barmak Heshmat.