MIT researchers working on an imaging system to read closed books

Researchers at the Massachusetts Institute of Technology (MIT) and their colleagues from Georgia Institute of Technology are working on an imaging system that will allow the reading of closed books.

They tested a prototype of the system on a stack of papers, each with one letter printed on it and the system was able to correctly identify the letters on the top nine sheets, says the study published in the journal Nature Communications.

Though still in the prototype stage, New York's Metropolitan Museum has shown interest in the technology, which can be used to examine the contents of antique books without damaging the object.

"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 MIT.

According to the researchers, the imaging system can also be used for analyzing materials in thin layers, like coatings on pharmaceuticals or machine parts.

For the system, MIT researchers developed the algorithms to acquire images from individual sheets in stacks of paper, and their Georgia Tech colleagues created the algorithm that interpreted the often distorted or incomplete images as individual letters.

"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," says Heshmat.

The system uses terahertz radiation, the band of electromagnetic radiation between microwaves and infrared light, which has several advantages over other types of waves that can penetrate surfaces, such as X-rays or sound waves. Terahertz frequency profiles can distinguish between ink and blank paper, in a way that X-rays cannot, and has much better depth resolution than ultrasound.

The prototype imaging system features a standard terahertz camera emitting ultrashort bursts of radiation. The camera also has built-in sensors so it's able to detect the reflections of the radiation.
From the reflections' time of arrival, the algorithm can gauge the distance to the individual pages of the book.

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.