Terahertz waves occupy the part of the spectrum between light and radio, specifically between infrared and millimetre waves. With wavelengths of 0.1-1mm, THz waves can be used like light or x-rays to create detailed images of solid objects. They have the useful property of passing easily through packaging and clothes, and since they are non-ionising they are safer than x-rays.
But THz waves can probe the content of objects as well as their shapes, thanks to their ability to respond to chemical properties. This is because their frequency range of 0.3-3THz matches the natural molecular vibrations of many common substances and biological materials.
Add these two properties together and you have a scanner that can not only detect a hidden package, but also show what is inside. New European research on THz waves could enable applications that include detecting tumours beneath the skin, a new and powerful kind of microscope for biological research, and quality control in semiconductor and pharmaceutical factories, as well as smart security scanners.