A group of researchers from MIT and Georgia Tech have built a device that can see through paper and distinguish ink from blank paper to determine what is written on the sheets. The prototype successfully identified letters printed on the top nine sheets of a stack of paper, and eventually the researchers hope to develop a system that can read closed books that have actual covers.

"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 research scientist at the MIT Media Lab and author on the new paper, published today in Nature Communications.

The imaging system works by using terahertz radiation, a frequency of light on the electromagnetic spectrum between microwaves and infrared light. Terahertz radiation is widely used in security screening, and it has a few advantages over other imaging techniques such as X-rays. Specifically, different chemicals absorb different amounts of terahertz radiation, creating a unique frequency signature for each material when the waves bounce back to a sensor, allowing the system to differentiate between ink and blank paper.

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Between pages of a book, there are air pockets that measure about 20 micrometers. The system uses short bursts of terahertz radiation waves to distinguish between individual sheets of paper by measuring the difference in the refractive index—the way a material bends light—between the air and the paper. The difference in light wave absorption between the ink and blank paper then allows the the system to produce an image of the written letters.

Multiple algorithms work in conjunction to create a legible image of the print. The first set, developed by MIT, produce the raw imaged based on the signals picked up by the device's sensor. An additional algorithm developed by Georgia Tech takes the often blurry and incomplete raw images and identifies the individual letters.

"It's actually kind of scary," Heshmat said of the letter-interpretation algorithm in a press release. "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."

The biggest challenge to fine-tuning this system so it could read every page of, say, a 200 page book, is canceling out the interference, or "noise," that is picked up by the sensor. Although most of the light waves are either absorbed by ink or paper, or bounced right back to the sensor, some of the radiation bounces back and forth between pages before making its way back to the detector. This interference prevents the current device from counting beyond 20 individual sheets, and it can only read the print on the top nine.

However, advances in terahertz radiation technologies, a relatively new field, could see new devices built in the near future that would be capable of reading pages much deeper in a stack of paper. By both increasing the power of the radiation source and improving the detectors that capture the rebounding signals, the research team is confident that "big promises for imaging new and exciting things" are just on the horizon.

Source: MIT

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