In the latest issue of Nature Communications, a group of researchers described 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. It is done with the help of terahertz radiation, the band of electromagnetic radiation between microwaves and infrared light, and the tiny gaps of air
between the pages of any closed book. This new system developed by researchers from MIT and Georgia Tech uses a standard terahertz camera to emit ultra short bursts of radiation and then measure how long it takes for that radiation to be reflected back. An algorithm then gauges the distance to each of the book's individual pages.
Terahertz radiation can also be emitted in such short bursts that the distance it has travelled can be gauged from the difference between its emission time and the time at which reflected radiation returns to a sensor. That gives it much better depth resolution than ultrasound.
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 and 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.
About Terahertz radiation
• Terahertz radiation can distinguish between ink and blank paper in a way that X-rays cannot.
• It also offers much better depth resolution than ultrasound does.
• 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.
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