In the latest issue of Nature Communications, the researchers describe a prototype of the system, which they tested 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 MIT researchers developed the algorithms that acquire images from individual sheets in stacks of paper, and the Georgia Tech researchers developed the algorithm that interprets the often distorted or incomplete images as individual letters.
"It's actually kind of scary," Heshmat says of the letter-interpretation algorithm. "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 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 radiation has been widely researched for use in security screening, because different chemicals absorb different frequencies of terahertz radiation to different degrees, yielding a distinctive frequency signature for each. By the same token, terahertz frequency profiles can distinguish between ink and blank paper, in a way that X-rays can't.
The system exploits the fact that trapped between the pages of a book are tiny air pockets only about 20 micrometers deep. The difference in refractive index—the degree to which they bend light—between the air and the paper means that the boundary between the two will reflect terahertz radiation back to a detector.
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.
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. Terahertz imaging is still a relatively young technology, however, and researchers are constantly working to improve both the accuracy of detectors and the power of the radiation sources, so deeper penetration should be possible.