WELL, THIS IS THE 21ST CENTURY, YOU KNOW: MIT And Georgia Tech Develop System To Read Closed Books Using Terahertz Radiation. “The system uses terahertz radiation, which unlike X-Rays, can distinguish between ink and paper. It also gives better depth resolution when compared to ultrasound. Paper and ink bend light at a different degree, which helps the system in distinguishing between them. The 20-micron deep air pockets between the pages help the system differentiate between the pages of a book.”