In their recent study, the team tested the system using a stack of papers with one letter printed on each and found that it could correctly identify those written on the top nine sheets.

The new system takes advantage of terahertz radiation – the band of electromagnetic radiation that lies between microwaves and infrared light on the electromagnetic spectrum. Although other wave types – such as X-rays – can also penetrate surfaces, the team chose to use terahertz radiation because it can differentiate between ink and blank paper in a way that X-rays cannot. This stems from the fact that different chemicals absorb different terahertz frequencies to varying degrees, giving each chemical – such as those used in ink and paper – a unique frequency signature.

MIT algorithms designed to capture images from each paper use this absorption difference to make the characters as clear as possible. Afterwards, algorithms developed by Georgia Tech were able to interpret the often-distorted images as letters.