Researchers at MIT just published a paper that outlines how a camera can read words and differentiate between pages in closed books.
When the radiation passes through the pages of a given book, it causes a pulse to bounce back towards the device at a certain frequency. Terahertz radiation reacts differently when it confronts different chemicals, so the camera uses a complicated algorithm to measure, interpret and translate those frequencies to distinguish the words. The camera is so sensitive that it can also measure how long it takes the radiation to reach the various gaps between pages, thereby determining which content falls on certain pages.
Source : https://www.businessinsider.fr/us/mit-camera-reads-closed-books-2016-9