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With MIT’s new imaging tech, now you can judge a book right through the cover

Ten years ago, MIT researchers demonstrated they could use terahertz imaging to read a letter inside a sealed FedEx envelope. Now a team from the MIT Media Lab has used terahertz radiation and a sophisticated image parsing algorithm to read the text on pages inside a stack up to nine sheets deep. It has implications for historical conservationists, real-time mail screening, and doubtless a bunch of other things that haven’t even been invented yet.

The new technique is like a cross between sonar and those much-maligned airport body scanners. It starts out using terahertz radiation, which is a band of EM radiation that has lower energy and longer wavelength than the visible and IR bands. Terahertz waves are still less than a millimeter long, peak-to-peak.

For this project, the researchers used terahertz-band radiation because it has several advantages over other types of waves (like X-rays or sound) that can penetrate surfaces. Just like how different chemicals polarize light differently, chemicals absorb frequencies of terahertz radiation to different degrees, throwing off a distinctive frequency signature for each. This is what makes them useful for distinguishing between C4 and tissue in the airport scanners. Terahertz frequency profiles can also distinguish between ink and blank paper, where X-rays can’t.

The system also exploits the principle of refraction. Trapped between the pages of a book are tiny air pockets only about 20 micrometers deep (presumably, filled with that new book smell). The difference in refractive index between the air and the paper means that the boundary between the two will reflect terahertz radiation back to a detector.

Terahertz radiation can also be used like sonar, because it can be emitted in extremely short bursts. The time between when the wavefront is emitted and when it’s reflected back to the sensor can accurately tell us how far it has traveled. This gives it much better depth resolution than ultrasound.

In the researchers’ setup, all these principles converge. A standard terahertz camera emits ultrashort bursts of radiation, and the camera’s built-in sensor detects the time to pingback. From the pingbacks’ time of arrival, the MIT researchers’ algorithm can gauge the distance to the individual pages of the book. Then, an algorithm developed by researchers from Georgia Tech interprets the often distorted or incomplete images as individual letters.

“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 Barmak Heshmat, a corresponding author of the new paper. Crumbling antique manuscripts could be imaged using this technology, without ever having to physically disturb the artifact.

And who knows what we might uncover, obscured beneath erasures and layers of ink and paint? The power of the letter-recognition algorithm is “actually kind of scary,” Heshmat says. “A lot of websites have these letter certifications [captchas] to make sure you’re not a robot, and this algorithm can get through a lot of them.”

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