Judging a book through its cover

MIT-Closed-Books-1_0
Barmak Heshmat

Original news release was issued by MIT, written by Larry Hardesty.

Thousands upon thousands of antique books are locked away in archival centres, withstanding the uncompromising tooth of time. Careful handling of the books — especially the rare ones — can be a pretty tricky business. Whenever there is a need to conduct obscure research on a book or to look up rare information, it is accompanied by the risk of damaging it. But now, researchers from MIT developed a prototype that can read closed books. They tested the system on a stack of papers, each with one letter printed on it. The system was able to correctly identify the letters on the top nine sheets.

“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,” says Barmak Heshmat, a research scientist at the MIT Media Lab and corresponding author on the new paper.

The prototype works with algorithms that acquire images from individual sheets of paper. A different set of algorithms then interprets the often distorted or incomplete images as individual letters. All of this is accomplished with the use of terahertz radiation — a relatively young technology still — which has several advantages over other types of waves that can penetrate surfaces, such as X-rays or sound waves. Different chemicals absorb different frequencies of terahertz radiation to different degrees, which yields a distinctive frequency signature for each.

Terahertz radiation can also be emitted in such short bursts that the distance it has traveled can be gauged from the difference between its emission time and the time at which reflected radiation returns to a sensor. In the researchers’ setup, a standard terahertz camera emits ultrashort bursts of radiation, and the camera’s built-in sensor detects their reflections. From the reflections’ time of arrival, the MIT researchers’ algorithm can gauge the distance to the individual pages of the book.

While most of the radiation is either absorbed or reflected by the book, some of it bounces around between pages before returning to the sensor, producing a spurious signal. The sensor’s electronics also produce a background hum. One of the tasks of the MIT researchers’ algorithm is to filter out all this “noise.”

“This work is one of the first to use these new tools along with advances in computational imaging to get at pictures of things we could never see with optical technologies. Now we can judge a book through its cover!” says Laura Waller, an associate professor of electrical engineering and computer science at the University of California at Berkeley.

At the moment, the algorithm can correctly deduce the distance from the camera to the top 20 pages in a stack, but past a depth of nine pages, the energy of the reflected signal is so low that the differences between frequency signatures are swamped by noise. However, the researchers are constantly working to improve both the accuracy of detectors and the power of the radiation sources, so deeper detection should be possible.

MIT-Closed-Books-2
Source: Barmak Heshmat

According to Heshmat, the system could also prove useful for analyzing any materials organized in thin layers, such as coatings on machine parts or pharmaceuticals. Furthermore, the work appears to have strong implications for research in industrial imaging, seismic imaging, and cultural heritage. If you wish to learn more about terahertz imaging, you can read the full paper here.

Michal Madaras

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