A variant on X-rays could scan the pages of a closed book according to MIT researchers. Tests already show the technique working on a bundle of nine sheets of paper.

The research is detailed in the snappily-titled Nature Communications article “Terahertz time-gated spectral imaging for content extraction through layered structures.”

It’s based around using T-rays, the letter coming from terahertz radiation, which is the band between microwave and infrared. The researchers have taken advantage of the fact that ink and paper absorb different frequencies of this radiation in different ways.

They also benefited from the way that even when book pages are closed, there’s a tiny air pocket between them. That’s enough for a T-ray camera to distinguish between paper and air and in turn distinguish individual pages.

After developing an algorithm to exploit these characteristics, the researchers tested it on a stack of nine sheets of paper, each 300 microns (thousandths of a millimeter) thick. That’s actually bulkier than printer paper or most book pages and is more akin to a cheap business card.

Each sheet had one capital letter written on it, around eight millimeters high, which is roughly equivalent to writing with a 24-point typeface. The scanning process produced an image for each sheet that made it possible to correctly identify the letter in each case. The image above is of a sheet with an L. The faint traces of H and Z are shadows cast from sheets on top of this, which is why the system has to be set up so precisely to distinguish the letters.

While this particular set-up would be of fairly limited use, the researchers say it was more of a proof of concept and that it should be possible to scale the principle up to deal with more sheets of paper and/or smaller text.

The most likely use of the technology would be to speed up bulk scanning of documents and books. The Metropolitan Museum of Art has also said its interested in using the concept to scan antique books that are too fragile to open. It could also be used for analyzing structures with lots of thin layers such as paint and coatings.

More excitable responses to the study have suggested spies using it to read intercepted letters without opening them, though the researchers note that this could be countered by using specific types of ink that couldn’t be “seen” this way.