Researchers, including Ramesh Raskar from Massachusetts Institute of Technology (MIT) in the US, tested a prototype of 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,” said Barmak Heshmat, a research scientist at MIT.
He said that the system could be used to analyse any materials organised in thin layers, such as coatings on machine parts or pharmaceuticals.
The researchers from MIT and Georgia Institute of Technology (Georgia Tech) in the US developed the algorithms that acquire images from individual sheets in stacks of paper, and interprets 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,” said 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 can not, and has much better depth resolution than ultrasound.
The system exploits the fact that between the pages of a book tiny air pockets are trapped about 20 micrometres 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 new system, a standard terahertz camera emits ultrashort bursts of radiation, and the camera’s built-in sensor detects their reflections.