2. Security and the military

2.1 Principle of terahertz imaging

Terahertz imaging systems can be divided into two families, depending on whether the aim is to record a terahertz image or film of an object or scene using its own thermal radiation (passive imaging) or by illuminating the scene with a terahertz source (active imaging). In both cases, the optical design of an observation system (video camera, still camera) is determined by the wavelengths involved, and therefore differs significantly from systems for visible or infrared optics. Indeed, optical systems, whatever the wavelength used (UV, VIS, IR, THz...), are subject to diffraction, and their spatial resolution is proportional to their numerical aperture. Two points A and B of the image formed on the photosensitive sensor are distinguishable if the...