Researchers have designed a prototype of an imaging system that can read a closed book, according to the latest issue of Nature Communications. The researchers tested 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, according to a 9 September press statement.
The system exploits the 20-micrometre deep air pockets trapped between the pages of a book. The difference in refractive index—the degree to which light is bent—between the air and the paper means that the boundary between the two will reflect terahertz radiation—the band of electromagnetic radiation between microwaves and infrared light—back to a detector. 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.
One of the tasks of the MIT researchers’ algorithm is to filter false signals resulting from some of the radiation bouncing around between pages before returning to the sensor and the background hum from the sensor’s electronics.
The algorithm can correctly gauge 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.